Link with -rdynamic instead of -Wl,-export-dynamic.

Recent versions of the OS X linker support this but follow the existing
OS X linker convention of using an underscore in the option name, i.e.,
-export_dynamic. Rather than changing our configure scripts to check for
that alternate spelling, it is simpler to just use the compiler's -rdynamic
option and let it deal with translating that to the appropriate linker
option. One potential disadvantage of this approach is that the compiler
will typically ignore -rdynamic on platforms where it is not supported, so
the HAVE_LINK_EXPORT_DYNAMIC in config.h will not necessarily show whether
that option has any effect or not. I don't see any in-tree uses of that
macro, so I'm assuming it is OK.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@187686 91177308-0d34-0410-b5e6-96231b3b80d8

projects/sample/autoconf/m4/link_options.m4

@@ -40,23 +40,24 @@ if test "$llvm_cv_link_use_r" = yes ; then
 ])
 
 #
-# Determine if the system can handle the -R option being passed to the linker.
+# Determine if the system can handle the -rdynamic option being passed
+# to the compiler.
 #
 # This macro is specific to LLVM.
 #
 AC_DEFUN([AC_LINK_EXPORT_DYNAMIC],
-[AC_CACHE_CHECK([for compiler -Wl,-export-dynamic option],
+[AC_CACHE_CHECK([for compiler -rdynamic option],
                 [llvm_cv_link_use_export_dynamic],
 [ AC_LANG_PUSH([C])
   oldcflags="$CFLAGS"
-  CFLAGS="$CFLAGS -Wl,-export-dynamic"
+  CFLAGS="$CFLAGS -rdynamic"
   AC_LINK_IFELSE([AC_LANG_PROGRAM([[]],[[]])],
     [llvm_cv_link_use_export_dynamic=yes],[llvm_cv_link_use_export_dynamic=no])
   CFLAGS="$oldcflags"
   AC_LANG_POP([C])
 ])
 if test "$llvm_cv_link_use_export_dynamic" = yes ; then
-  AC_DEFINE([HAVE_LINK_EXPORT_DYNAMIC],[1],[Define if you can use -Wl,-export-dynamic.])
+  AC_DEFINE([HAVE_LINK_EXPORT_DYNAMIC],[1],[Define if you can use -rdynamic.])
   fi
 ])