Fix non-Xcode build on Mac with clang.

src/globals.h

Index: src/globals.h
diff --git a/src/globals.h b/src/globals.h
index 58832df45d56541aac8da031208a221d286b667c..2f78e4fc612c1d1b2a6fcf47ee710ce3a55813d1 100644
--- a/src/globals.h
+++ b/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