Switch GYP to use LLVM-GCC when using Xcode.

R=jkummerow@chromium.org

Review URL: https://codereview.chromium.org/12228004

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@13600 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

build/standalone.gypi

@@ -192,7 +192,7 @@
           'GCC_SYMBOLS_PRIVATE_EXTERN': 'YES',      # -fvisibility=hidden
           'GCC_THREADSAFE_STATICS': 'NO',           # -fno-threadsafe-statics
           'GCC_TREAT_WARNINGS_AS_ERRORS': 'YES',    # -Werror
-          'GCC_VERSION': '4.2',
+          'GCC_VERSION': 'com.apple.compilers.llvmgcc42',
           'GCC_WARN_ABOUT_MISSING_NEWLINE': 'YES',  # -Wnewline-eof
           # MACOSX_DEPLOYMENT_TARGET maps to -mmacosx-version-min
           'MACOSX_DEPLOYMENT_TARGET': '<(mac_deployment_target)',