Bring the script out of the dark ages and into modern autoconfness.

llvm-svn: 16415
diff --git a/llvm/autoconf/m4/cxx_namespaces.m4 b/llvm/autoconf/m4/cxx_namespaces.m4
index 756e378..d8d650e 100644
--- a/llvm/autoconf/m4/cxx_namespaces.m4
+++ b/llvm/autoconf/m4/cxx_namespaces.m4
@@ -4,12 +4,13 @@
 AC_DEFUN([AC_CXX_NAMESPACES],
 [AC_CACHE_CHECK(whether the compiler implements namespaces,
 ac_cv_cxx_namespaces,
-[AC_LANG_SAVE
- AC_LANG_CPLUSPLUS
- AC_TRY_COMPILE([namespace Outer { namespace Inner { int i = 0; }}],
-                [using namespace Outer::Inner; return i;],
- ac_cv_cxx_namespaces=yes, ac_cv_cxx_namespaces=no)
- AC_LANG_RESTORE
+[AC_LANG_PUSH([C++])
+ AC_COMPILE_IFELSE([AC_LANG_PROGRAM(
+   [[namespace Outer { namespace Inner { int i = 0; }}]],
+   [[using namespace Outer::Inner; return i;]])], 
+   ac_cv_cxx_namespaces=yes, 
+   ac_cv_cxx_namespaces=no)
+ AC_LANG_POP([C++])
 ])
 if test "$ac_cv_cxx_namespaces" = yes; then
   AC_DEFINE(HAVE_NAMESPACES,,[define if the compiler implements namespaces])