Use third_party clang on Mac.

build/standalone.gypi

 # Copyright 2012 the V8 project authors. All rights reserved.
 # Redistribution and use in source and binary forms, with or without
 # modification, are permitted provided that the following conditions are
 # met:
 #
 #     * Redistributions of source code must retain the above copyright
 #       notice, this list of conditions and the following disclaimer.
 #     * Redistributions in binary form must reproduce the above
 #       copyright notice, this list of conditions and the following
 #       disclaimer in the documentation and/or other materials provided
@@ skipping 750 matching lines @@
           ],
         },
         'conditions': [
           ['werror==""', {
             'xcode_settings': {'GCC_TREAT_WARNINGS_AS_ERRORS': 'NO'},
           }, {
             'xcode_settings': {'GCC_TREAT_WARNINGS_AS_ERRORS': 'YES'},
           }],
           ['clang==1', {
             'xcode_settings': {
+              'CC': '<(clang_dir)/bin/clang',
+              'LDPLUSPLUS': '<(clang_dir)/bin/clang++',
               'GCC_VERSION': 'com.apple.compilers.llvm.clang.1_0',
               'CLANG_CXX_LANGUAGE_STANDARD': 'gnu++0x',  # -std=gnu++0x
+              'CLANG_CXX_LIBRARY': 'libc++',
             },
           }],
         ],
         'target_conditions': [
           ['_type!="static_library"', {
             'xcode_settings': {'OTHER_LDFLAGS': ['-Wl,-search_paths_first']},
           }],
         ],  # target_conditions
       },  # target_defaults
     }],  # OS=="mac"
@@ skipping 256 matching lines @@
     ['use_goma==1 and ("<(GENERATOR)"=="ninja" or clang==1)', {
       'make_global_settings': [
        ['CC_wrapper', '<(gomadir)/gomacc'],
        ['CXX_wrapper', '<(gomadir)/gomacc'],
        ['CC.host_wrapper', '<(gomadir)/gomacc'],
        ['CXX.host_wrapper', '<(gomadir)/gomacc'],
       ],
     }],
   ],
 }