Prevent SIGSEGV using both python and python3

    $ vim --cmd 'py3 print("test")' --cmd 'py print("test")'
    $ vim --cmd 'py print("test")' --cmd 'py3 print("test")'

It causes SIGSEGV. Technically, it would work with special Python2 built
and Python3 built. However Python2 in OS X System and Python3 in
Homebrew don't have the capability at all. This patch prevents SIGSEGV
and make Vim can track the situation properly.

    $ vim --cmd 'py print("test")' --cmd 'py3 print("test")'
    E837: This Vim cannot execute :py3 after using :python
    E263: Sorry, this command is disabled, the Python library could not be loaded.

Author: splhack

src/auto/configure

@@ -6447,6 +6447,7 @@ $as_echo_n "checking whether we can do without RTLD_GLOBAL for Python... " >&6;
   CFLAGS="$CFLAGS $PYTHON_CFLAGS"
   libs_save=$LIBS
     LIBS="-ldl $LIBS"
+  if test "x$MACOSX" != "xyes"; then
   if test "$cross_compiling" = yes; then :
   { { $as_echo "$as_me:${as_lineno-$LINENO}: error: in \`$ac_pwd':" >&5
 $as_echo "$as_me: error: in \`$ac_pwd':" >&2;}
@@ -6568,6 +6569,7 @@ rm -f core *.core core.conftest.* gmon.out bb.out conftest$ac_exeext \
   conftest.$ac_objext conftest.beam conftest.$ac_ext
 fi
 
+  fi
 
   CFLAGS=$cflags_save
   LIBS=$libs_save

src/configure.in

@@ -1520,6 +1520,7 @@ if test "$python_ok" = yes && test "$python3_ok" = yes; then
   libs_save=$LIBS
   dnl -ldl must go first to make this work on Archlinux (Roland Puntaier)
   LIBS="-ldl $LIBS"
+  if test "x$MACOSX" != "xyes"; then
   AC_RUN_IFELSE([AC_LANG_SOURCE([
     #include <dlfcn.h>
     /* If this program fails, then RTLD_GLOBAL is needed.
@@ -1601,6 +1602,7 @@ if test "$python_ok" = yes && test "$python3_ok" = yes; then
       return !not_needed;
     }])],
     [AC_MSG_RESULT(yes);AC_DEFINE(PY3_NO_RTLD_GLOBAL)], [AC_MSG_RESULT(no)])
+  fi
 
   CFLAGS=$cflags_save
   LIBS=$libs_save