diff --git a/Misc/NEWS.d/next/Build/2018-10-16-12-22-36.bpo-28015.ylSgFh.rst b/Misc/NEWS.d/next/Build/2018-10-16-12-22-36.bpo-28015.ylSgFh.rst
new file mode 100644
index 00000000000..5a7a43c7730
--- /dev/null
+++ b/Misc/NEWS.d/next/Build/2018-10-16-12-22-36.bpo-28015.ylSgFh.rst
@@ -0,0 +1 @@
+Have --with-lto works correctly with clang.
diff --git a/aclocal.m4 b/aclocal.m4
index 030e6877de9..94a2dd6d3eb 100644
--- a/aclocal.m4
+++ b/aclocal.m4
@@ -13,7 +13,7 @@
 
 m4_ifndef([AC_CONFIG_MACRO_DIRS], [m4_defun([_AM_CONFIG_MACRO_DIRS], [])m4_defun([AC_CONFIG_MACRO_DIRS], [_AM_CONFIG_MACRO_DIRS($@)])])
 dnl pkg.m4 - Macros to locate and utilise pkg-config.   -*- Autoconf -*-
-dnl serial 11 (pkg-config-0.29.1)
+dnl serial 11 (pkg-config-0.29)
 dnl
 dnl Copyright © 2004 Scott James Remnant <scott@netsplit.com>.
 dnl Copyright © 2012-2015 Dan Nicholson <dbn.lists@gmail.com>
@@ -55,7 +55,7 @@ dnl
 dnl See the "Since" comment for each macro you use to see what version
 dnl of the macros you require.
 m4_defun([PKG_PREREQ],
-[m4_define([PKG_MACROS_VERSION], [0.29.1])
+[m4_define([PKG_MACROS_VERSION], [0.29])
 m4_if(m4_version_compare(PKG_MACROS_VERSION, [$1]), -1,
     [m4_fatal([pkg.m4 version $1 or higher is required but ]PKG_MACROS_VERSION[ found])])
 ])dnl PKG_PREREQ
diff --git a/configure b/configure
index 2f45c17852e..3d66c1cb1c2 100755
--- a/configure
+++ b/configure
@@ -671,16 +671,18 @@ BASECFLAGS
 CFLAGS_ALIASING
 OPT
 LLVM_PROF_FOUND
-target_os
-target_vendor
-target_cpu
-target
 LLVM_PROFDATA
 LLVM_PROF_ERR
 LLVM_PROF_FILE
 LLVM_PROF_MERGER
 PGO_PROF_USE_FLAG
 PGO_PROF_GEN_FLAG
+LLVM_AR_FOUND
+target_os
+target_vendor
+target_cpu
+target
+LLVM_AR
 DEF_MAKE_RULE
 DEF_MAKE_ALL_RULE
 ABIFLAGS
@@ -6388,6 +6390,26 @@ else
   DEF_MAKE_RULE="all"
 fi
 
+# Make llvm-relatec checks work on systems where llvm tools are not installed with their
+# normal names in the default $PATH (ie: Ubuntu).  They exist under the
+# non-suffixed name in their versioned llvm directory.
+
+llvm_bin_dir=''
+llvm_path="${PATH}"
+if test "${CC}" = "clang"
+then
+  clang_bin=`which clang`
+  # Some systems install clang elsewhere as a symlink to the real path
+  # which is where the related llvm tools are located.
+  if test -L "${clang_bin}"
+  then
+    clang_dir=`dirname "${clang_bin}"`
+    clang_bin=`readlink "${clang_bin}"`
+    llvm_bin_dir="${clang_dir}/"`dirname "${clang_bin}"`
+    llvm_path="${llvm_path}${PATH_SEPARATOR}${llvm_bin_dir}"
+  fi
+fi
+
 # Enable LTO flags
 { $as_echo "$as_me:${as_lineno-$LINENO}: checking for --with-lto" >&5
 $as_echo_n "checking for --with-lto... " >&6; }
@@ -6413,65 +6435,8 @@ fi
 if test "$Py_LTO" = 'true' ; then
   case $CC in
     *clang*)
-      case $ac_sys_system in
-        Darwin*)
-          # Any changes made here should be reflected in the GCC+Darwin case below
-          LTOFLAGS="-flto -Wl,-export_dynamic"
-          ;;
-        *)
-          LTOFLAGS="-flto"
-          ;;
-      esac
-      ;;
-    *gcc*)
-      case $ac_sys_system in
-        Darwin*)
-          LTOFLAGS="-flto -Wl,-export_dynamic"
-          ;;
-        *)
-          LTOFLAGS="-flto -fuse-linker-plugin -ffat-lto-objects -flto-partition=none"
-          ;;
-      esac
-      ;;
-  esac
 
-  if test "$ac_cv_prog_cc_g" = "yes"
-  then
-      # bpo-30345: Add -g to LDFLAGS when compiling with LTO
-      # to get debug symbols.
-      LTOFLAGS="$LTOFLAGS -g"
-  fi
-
-  CFLAGS="$CFLAGS $LTOFLAGS"
-  LDFLAGS="$LDFLAGS $LTOFLAGS"
-fi
-
-# Enable PGO flags.
-
-
-
-
-
-# Make this work on systems where llvm tools are not installed with their
-# normal names in the default $PATH (ie: Ubuntu).  They exist under the
-# non-suffixed name in their versioned llvm directory.
-llvm_bin_dir=''
-llvm_path="${PATH}"
-if test "${CC}" = "clang"
-then
-  clang_bin=`which clang`
-  # Some systems install clang elsewhere as a symlink to the real path
-  # which is where the related llvm tools are located.
-  if test -L "${clang_bin}"
-  then
-    clang_dir=`dirname "${clang_bin}"`
-    clang_bin=`readlink "${clang_bin}"`
-    llvm_bin_dir="${clang_dir}/"`dirname "${clang_bin}"`
-    llvm_path="${llvm_path}${PATH_SEPARATOR}${llvm_bin_dir}"
-  fi
-fi
-
-{ $as_echo "$as_me:${as_lineno-$LINENO}: checking target system type" >&5
+      { $as_echo "$as_me:${as_lineno-$LINENO}: checking target system type" >&5
 $as_echo_n "checking target system type... " >&6; }
 if ${ac_cv_target+:} false; then :
   $as_echo_n "(cached) " >&6
@@ -6510,6 +6475,163 @@ test -n "$target_alias" &&
   test "$program_prefix$program_suffix$program_transform_name" = \
     NONENONEs,x,x, &&
   program_prefix=${target_alias}-
+# Extract the first word of "$target_alias-llvm-ar", so it can be a program name with args.
+set dummy $target_alias-llvm-ar; ac_word=$2
+{ $as_echo "$as_me:${as_lineno-$LINENO}: checking for $ac_word" >&5
+$as_echo_n "checking for $ac_word... " >&6; }
+if ${ac_cv_path_LLVM_AR+:} false; then :
+  $as_echo_n "(cached) " >&6
+else
+  case $LLVM_AR in
+  [\\/]* | ?:[\\/]*)
+  ac_cv_path_LLVM_AR="$LLVM_AR" # Let the user override the test with a path.
+  ;;
+  *)
+  as_save_IFS=$IFS; IFS=$PATH_SEPARATOR
+for as_dir in ${llvm_path}
+do
+  IFS=$as_save_IFS
+  test -z "$as_dir" && as_dir=.
+    for ac_exec_ext in '' $ac_executable_extensions; do
+  if as_fn_executable_p "$as_dir/$ac_word$ac_exec_ext"; then
+    ac_cv_path_LLVM_AR="$as_dir/$ac_word$ac_exec_ext"
+    $as_echo "$as_me:${as_lineno-$LINENO}: found $as_dir/$ac_word$ac_exec_ext" >&5
+    break 2
+  fi
+done
+  done
+IFS=$as_save_IFS
+
+  ;;
+esac
+fi
+LLVM_AR=$ac_cv_path_LLVM_AR
+if test -n "$LLVM_AR"; then
+  { $as_echo "$as_me:${as_lineno-$LINENO}: result: $LLVM_AR" >&5
+$as_echo "$LLVM_AR" >&6; }
+else
+  { $as_echo "$as_me:${as_lineno-$LINENO}: result: no" >&5
+$as_echo "no" >&6; }
+fi
+
+
+if test -z "$ac_cv_path_LLVM_AR"; then
+  if test "$build" = "$target"; then
+    ac_pt_LLVM_AR=$LLVM_AR
+    # Extract the first word of "llvm-ar", so it can be a program name with args.
+set dummy llvm-ar; ac_word=$2
+{ $as_echo "$as_me:${as_lineno-$LINENO}: checking for $ac_word" >&5
+$as_echo_n "checking for $ac_word... " >&6; }
+if ${ac_cv_path_ac_pt_LLVM_AR+:} false; then :
+  $as_echo_n "(cached) " >&6
+else
+  case $ac_pt_LLVM_AR in
+  [\\/]* | ?:[\\/]*)
+  ac_cv_path_ac_pt_LLVM_AR="$ac_pt_LLVM_AR" # Let the user override the test with a path.
+  ;;
+  *)
+  as_save_IFS=$IFS; IFS=$PATH_SEPARATOR
+for as_dir in ${llvm_path}
+do
+  IFS=$as_save_IFS
+  test -z "$as_dir" && as_dir=.
+    for ac_exec_ext in '' $ac_executable_extensions; do
+  if as_fn_executable_p "$as_dir/$ac_word$ac_exec_ext"; then
+    ac_cv_path_ac_pt_LLVM_AR="$as_dir/$ac_word$ac_exec_ext"
+    $as_echo "$as_me:${as_lineno-$LINENO}: found $as_dir/$ac_word$ac_exec_ext" >&5
+    break 2
+  fi
+done
+  done
+IFS=$as_save_IFS
+
+  test -z "$ac_cv_path_ac_pt_LLVM_AR" && ac_cv_path_ac_pt_LLVM_AR="''"
+  ;;
+esac
+fi
+ac_pt_LLVM_AR=$ac_cv_path_ac_pt_LLVM_AR
+if test -n "$ac_pt_LLVM_AR"; then
+  { $as_echo "$as_me:${as_lineno-$LINENO}: result: $ac_pt_LLVM_AR" >&5
+$as_echo "$ac_pt_LLVM_AR" >&6; }
+else
+  { $as_echo "$as_me:${as_lineno-$LINENO}: result: no" >&5
+$as_echo "no" >&6; }
+fi
+
+    LLVM_AR=$ac_pt_LLVM_AR
+  else
+    LLVM_AR="''"
+  fi
+else
+  LLVM_AR="$ac_cv_path_LLVM_AR"
+fi
+
+
+      if test -n "${LLVM_AR}" -a -x "${LLVM_AR}"
+      then
+        LLVM_AR_FOUND="found"
+      else
+        LLVM_AR_FOUND="not-found"
+      fi
+      if test "$ac_sys_system" = "Darwin" -a "${LLVM_AR_FOUND}" = "not-found"
+      then
+        found_llvm_ar=`/usr/bin/xcrun -find llvm-ar 2>/dev/null`
+        if test -n "${found_llvm_ar}"
+        then
+          LLVM_AR='/usr/bin/xcrun llvm-ar'
+          LLVM_AR_FOUND=found
+          { $as_echo "$as_me:${as_lineno-$LINENO}: llvm-ar found via xcrun: ${LLVM_AR}" >&5
+$as_echo "$as_me: llvm-ar found via xcrun: ${LLVM_AR}" >&6;}
+        fi
+      fi
+      if test $LLVM_AR_FOUND = not-found
+      then
+        LLVM_PROFR_ERR=yes
+        as_fn_error $? "llvm-ar is required for a --with-lto build with clang but could not be found." "$LINENO" 5
+      else
+        LLVM_AR_ERR=no
+      fi
+      AR="${LLVM_AR}"
+      case $ac_sys_system in
+        Darwin*)
+          # Any changes made here should be reflected in the GCC+Darwin case below
+          LTOFLAGS="-flto -Wl,-export_dynamic"
+          ;;
+        *)
+          LTOFLAGS="-flto"
+          ;;
+      esac
+      ;;
+    *gcc*)
+      case $ac_sys_system in
+        Darwin*)
+          LTOFLAGS="-flto -Wl,-export_dynamic"
+          ;;
+        *)
+          LTOFLAGS="-flto -fuse-linker-plugin -ffat-lto-objects -flto-partition=none"
+          ;;
+      esac
+      ;;
+  esac
+
+  if test "$ac_cv_prog_cc_g" = "yes"
+  then
+      # bpo-30345: Add -g to LDFLAGS when compiling with LTO
+      # to get debug symbols.
+      LTOFLAGS="$LTOFLAGS -g"
+  fi
+
+  BASECFLAGS="$BASECFLAGS $LTOFLAGS"
+  LDFLAGS="$LDFLAGS $LTOFLAGS"
+fi
+
+# Enable PGO flags.
+
+
+
+
+
+
 # Extract the first word of "$target_alias-llvm-profdata", so it can be a program name with args.
 set dummy $target_alias-llvm-profdata; ac_word=$2
 { $as_echo "$as_me:${as_lineno-$LINENO}: checking for $ac_word" >&5
diff --git a/configure.ac b/configure.ac
index f1b47dbf4f8..d1502dfef1f 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1265,6 +1265,26 @@ else
   DEF_MAKE_RULE="all"
 fi
 
+# Make llvm-relatec checks work on systems where llvm tools are not installed with their
+# normal names in the default $PATH (ie: Ubuntu).  They exist under the
+# non-suffixed name in their versioned llvm directory.
+
+llvm_bin_dir=''
+llvm_path="${PATH}"
+if test "${CC}" = "clang"
+then
+  clang_bin=`which clang`
+  # Some systems install clang elsewhere as a symlink to the real path
+  # which is where the related llvm tools are located.
+  if test -L "${clang_bin}"
+  then
+    clang_dir=`dirname "${clang_bin}"`
+    clang_bin=`readlink "${clang_bin}"`
+    llvm_bin_dir="${clang_dir}/"`dirname "${clang_bin}"`
+    llvm_path="${llvm_path}${PATH_SEPARATOR}${llvm_bin_dir}"
+  fi
+fi
+
 # Enable LTO flags
 AC_MSG_CHECKING(for --with-lto)
 AC_ARG_WITH(lto, AS_HELP_STRING([--with-lto], [Enable Link Time Optimization in any build. Disabled by default.]),
@@ -1281,6 +1301,33 @@ fi],
 if test "$Py_LTO" = 'true' ; then
   case $CC in
     *clang*)
+      AC_SUBST(LLVM_AR)
+      AC_PATH_TARGET_TOOL(LLVM_AR, llvm-ar, '', ${llvm_path})
+      AC_SUBST(LLVM_AR_FOUND)
+      if test -n "${LLVM_AR}" -a -x "${LLVM_AR}"
+      then
+        LLVM_AR_FOUND="found"
+      else
+        LLVM_AR_FOUND="not-found"
+      fi
+      if test "$ac_sys_system" = "Darwin" -a "${LLVM_AR_FOUND}" = "not-found"
+      then
+        found_llvm_ar=`/usr/bin/xcrun -find llvm-ar 2>/dev/null`
+        if test -n "${found_llvm_ar}"
+        then
+          LLVM_AR='/usr/bin/xcrun llvm-ar'
+          LLVM_AR_FOUND=found
+          AC_MSG_NOTICE([llvm-ar found via xcrun: ${LLVM_AR}])
+        fi
+      fi
+      if test $LLVM_AR_FOUND = not-found
+      then
+        LLVM_PROFR_ERR=yes
+        AC_MSG_ERROR([llvm-ar is required for a --with-lto build with clang but could not be found.])
+      else
+        LLVM_AR_ERR=no
+      fi
+      AR="${LLVM_AR}"
       case $ac_sys_system in
         Darwin*)
           # Any changes made here should be reflected in the GCC+Darwin case below
@@ -1310,7 +1357,7 @@ if test "$Py_LTO" = 'true' ; then
       LTOFLAGS="$LTOFLAGS -g"
   fi
 
-  CFLAGS="$CFLAGS $LTOFLAGS"
+  BASECFLAGS="$BASECFLAGS $LTOFLAGS"
   LDFLAGS="$LDFLAGS $LTOFLAGS"
 fi
 
@@ -1320,24 +1367,6 @@ AC_SUBST(PGO_PROF_USE_FLAG)
 AC_SUBST(LLVM_PROF_MERGER)
 AC_SUBST(LLVM_PROF_FILE)
 AC_SUBST(LLVM_PROF_ERR)
-# Make this work on systems where llvm tools are not installed with their
-# normal names in the default $PATH (ie: Ubuntu).  They exist under the
-# non-suffixed name in their versioned llvm directory.
-llvm_bin_dir=''
-llvm_path="${PATH}"
-if test "${CC}" = "clang"
-then
-  clang_bin=`which clang`
-  # Some systems install clang elsewhere as a symlink to the real path
-  # which is where the related llvm tools are located.
-  if test -L "${clang_bin}"
-  then
-    clang_dir=`dirname "${clang_bin}"`
-    clang_bin=`readlink "${clang_bin}"`
-    llvm_bin_dir="${clang_dir}/"`dirname "${clang_bin}"`
-    llvm_path="${llvm_path}${PATH_SEPARATOR}${llvm_bin_dir}"
-  fi
-fi
 AC_SUBST(LLVM_PROFDATA)
 AC_PATH_TARGET_TOOL(LLVM_PROFDATA, llvm-profdata, '', ${llvm_path})
 AC_SUBST(LLVM_PROF_FOUND)