Issue #11888: Use system log2() when available

I expect the system libc to use more accurate functions than Python. The GNU
libc uses for example FYL2X and FYL2XP1 hardware instructions on Intel FPU.

Modules/mathmodule.c

@@ -602,6 +602,9 @@ m_log2(double x)
     }
 
     if (x > 0.0) {
+#ifdef HAVE_LOG2
+        return log2(x);
+#else
         double m;
         int e;
         m = frexp(x, &e);
@@ -617,6 +620,7 @@ m_log2(double x)
         else {
             return log(m) / log(2.0) + e;
         }
+#endif
     }
     else if (x == 0.0) {
         errno = EDOM;

configure

@@ -11864,7 +11864,7 @@ _ACEOF
 fi
 done
 
-for ac_func in hypot lgamma log1p round tgamma
+for ac_func in hypot lgamma log1p log2 round tgamma
 do :
   as_ac_var=`$as_echo "ac_cv_func_$ac_func" | $as_tr_sh`
 ac_fn_c_check_func "$LINENO" "$ac_func" "$as_ac_var"

configure.in

@@ -2512,7 +2512,7 @@ AC_CHECK_FUNCS(alarm accept4 setitimer getitimer bind_textdomain_codeset chown \
  futimens futimes \
  gai_strerror getgroups getlogin getloadavg getpeername getpgid getpid \
  getpriority getresuid getresgid getpwent getspnam getspent getsid getwd \
- initgroups kill killpg lchmod lchown lockf linkat lstat lutimes mbrtowc mkdirat mkfifo \
+ initgroups kill killpg lchmod lchown lockf log2 linkat lstat lutimes mbrtowc mkdirat mkfifo \
  mkfifoat mknod mknodat mktime mremap nice openat pathconf pause plock poll \
  posix_fallocate posix_fadvise pread \
  pthread_init pthread_kill putenv pwrite readlink readlinkat readv realpath renameat \
@@ -3368,7 +3368,7 @@ LIBS_SAVE=$LIBS
 LIBS="$LIBS $LIBM"
 
 AC_CHECK_FUNCS([acosh asinh atanh copysign erf erfc expm1 finite gamma])
-AC_CHECK_FUNCS([hypot lgamma log1p round tgamma])
+AC_CHECK_FUNCS([hypot lgamma log1p log2 round tgamma])
 AC_CHECK_DECLS([isinf, isnan, isfinite], [], [], [[#include <math.h>]])
 
 # On FreeBSD 6.2, it appears that tanh(-0.) returns 0. instead of

pyconfig.h.in

@@ -476,6 +476,9 @@
 /* Define to 1 if you have the `log1p' function. */
 #undef HAVE_LOG1P
 
+/* Define to 1 if you have the `log2' function. */
+#undef HAVE_LOG2
+
 /* Define this if you have the type long double. */
 #undef HAVE_LONG_DOUBLE