bpo-40684: Fix make install for platlibdir=lib64 (GH-20736)

"make install" now uses the PLATLIBDIR variable for the destination
lib-dynload/ directory when ./configure --with-platlibdir is used.

Update --with-platlibdir comment in configure.

Makefile.pre.in

@@ -148,7 +148,7 @@ SCRIPTDIR=	$(prefix)/$(PLATLIBDIR)
 ABIFLAGS=	@ABIFLAGS@
 
 # Detailed destination directories
-BINLIBDEST=	$(LIBDIR)/python$(VERSION)
+BINLIBDEST=	@BINLIBDEST@
 LIBDEST=	$(SCRIPTDIR)/python$(VERSION)
 INCLUDEPY=	$(INCLUDEDIR)/python$(LDVERSION)
 CONFINCLUDEPY=	$(CONFINCLUDEDIR)/python$(LDVERSION)

Misc/NEWS.d/next/Build/2020-06-08-19-57-05.bpo-40684.WIY2-i.rst

@@ -0,0 +1,2 @@
+``make install`` now uses the ``PLATLIBDIR`` variable for the destination
+``lib-dynload/`` directory when ``./configure --with-platlibdir`` is used.

configure

@@ -632,6 +632,7 @@ THREADHEADERS
 LIBPL
 PY_ENABLE_SHARED
 PLATLIBDIR
+BINLIBDEST
 LIBPYTHON
 EXT_SUFFIX
 ALT_SOABI
@@ -15334,7 +15335,11 @@ else
 fi
 
 
-# Check for --with-libdir-name
+
+BINLIBDEST='$(LIBDIR)/python$(VERSION)'
+
+
+# Check for --with-platlibdir
 # /usr/$LIDIRNAME/python$VERSION
 
 PLATLIBDIR="lib"
@@ -15353,6 +15358,7 @@ then
   { $as_echo "$as_me:${as_lineno-$LINENO}: result: yes" >&5
 $as_echo "yes" >&6; }
   PLATLIBDIR="$withval"
+  BINLIBDEST='${exec_prefix}/${PLATLIBDIR}/python$(VERSION)'
 else
   { $as_echo "$as_me:${as_lineno-$LINENO}: result: no" >&5
 $as_echo "no" >&6; }

configure.ac

@@ -4770,7 +4770,11 @@ else
 fi
 
 
-# Check for --with-libdir-name
+AC_SUBST(BINLIBDEST)
+BINLIBDEST='$(LIBDIR)/python$(VERSION)'
+
+
+# Check for --with-platlibdir
 # /usr/$LIDIRNAME/python$VERSION
 AC_SUBST(PLATLIBDIR)
 PLATLIBDIR="lib"
@@ -4787,6 +4791,7 @@ if test -n "$withval" -a "$withval" != yes -a "$withval" != no
 then
   AC_MSG_RESULT(yes)
   PLATLIBDIR="$withval"
+  BINLIBDEST='${exec_prefix}/${PLATLIBDIR}/python$(VERSION)'
 else
   AC_MSG_RESULT(no)
 fi],