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

test/mac/gyptest-xcode-gcc.py

 #!/usr/bin/env python
 
 # 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.
 
 """
 Verifies that xcode-style GCC_... settings are handled properly.
 """
 
@@ skipping 13 matching lines @@
 
   # List of targets that'll pass. It expects targets of the same name with
   # '-fail' appended that'll fail to build.
   targets = [
     'warn_about_missing_newline',
   ]
 
   # clang doesn't warn on invalid offsetofs, it silently ignores
   # -Wno-invalid-offsetof.
   # TODO(thakis): This isn't really the right way to detect the compiler,
-  # `which cc` detects what make ends up using, and Xcode has some embedded
-  # compiler, but it's a reliable proxy at least on the bots.
-  if os.readlink('/usr/bin/cc') != 'clang':
+  # Xcode has some embedded compiler, but it's a reliable proxy at least on
+  # the bots. The compiler is forced to gcc/g++ in the gyp file in a
+  # make_global_settings section for ninja and make.
+  if test.format != 'xcode' or os.readlink('/usr/bin/cc') != 'clang':
     targets.append('warn_about_invalid_offsetof_macro')
 
   for target in targets:
     test.build('test.gyp', target, chdir=CHDIR)
     test.built_file_must_exist(target, chdir=CHDIR)
     fail_target = target + '-fail'
     test.build('test.gyp', fail_target, chdir=CHDIR, status=None,
                stderr=None, match=IgnoreOutput)
     test.built_file_must_not_exist(fail_target, chdir=CHDIR)
 
   test.pass_test()