ninja: use clang by default on Mac, matching recent Xcodes

test/mac/xcode-gcc/test.gyp

 # Copyright (c) 2012 Google Inc. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 {
+  'make_global_settings': [
+    ['CC', '/usr/bin/gcc'],
+    ['CXX', '/usr/bin/g++'],
+  ],
+
   'target_defaults': {
     'xcode_settings': {
       'GCC_TREAT_WARNINGS_AS_ERRORS': 'YES',
     },
   },
 
   'variables': {
     # Non-failing tests should check that these trivial files in every language
     # still compile correctly.
     'valid_sources': [
@@ skipping 36 matching lines @@
     {
       'target_name': 'warn_about_missing_newline-fail',
       'type': 'executable',
       'sources': [ 'warn_about_missing_newline.c', ],
       'xcode_settings': {
         'GCC_WARN_ABOUT_MISSING_NEWLINE': 'YES',
       },
     },
   ],
 }