Fix non-Xcode build on Mac with clang.

TBR=svenpanne@chromium.org

Review URL: https://codereview.chromium.org/22793008

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@16236 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

src/globals.h

@@ -333,12 +333,7 @@ F FUNCTION_CAST(Address addr) {
 // Compiler feature detection.
 #if defined(__clang__)
 
-// Compatibility with older clang versions.
-# ifndef __has_extension
-# define __has_extension __has_feature
-# endif
-
-# if __has_extension(cxx_override_control)
+# if __has_feature(cxx_override_control)
 #  define V8_HAVE_CXX11_FINAL
 #  define V8_HAVE_CXX11_OVERRIDE
 # endif