Add support for GCC_WARN_ABOUT_INVALID_OFFSETOF_MACRO to xcode emulation.

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.
+{
+  'target_defaults': {
+    'xcode_settings': {
+      'GCC_TREAT_WARNINGS_AS_ERRORS': 'YES',
+    },
+  },
+
+  # Targets come in pairs: 'foo' and 'foo-fail', with the former building with
+  # no warnings and the latter not.
+  'targets': [
+    # GCC_WARN_ABOUT_INVALID_OFFSETOF_MACRO (default: YES):
+    {
+      'target_name': 'warn_about_invalid_offsetof_macro',
+      'type': 'executable',
+      'sources': [ 'warn_about_invalid_offsetof_macro.cc', ],
+      'xcode_settings': {
+        'GCC_WARN_ABOUT_INVALID_OFFSETOF_MACRO': 'NO',
+      },
+    },
+    {
+      'target_name': 'warn_about_invalid_offsetof_macro-fail',
+      'type': 'executable',
+      'sources': [ 'warn_about_invalid_offsetof_macro.cc', ],
+    },
+    # GCC_WARN_ABOUT_MISSING_NEWLINE (default: NO):
+    {
+      'target_name': 'warn_about_missing_newline',
+      'type': 'executable',
+      'sources': [ 'warn_about_missing_newline.c', ],
+    },
+    {
+      'target_name': 'warn_about_missing_newline-fail',
+      'type': 'executable',
+      'sources': [ 'warn_about_missing_newline.c', ],
+      'xcode_settings': {
+        'GCC_WARN_ABOUT_MISSING_NEWLINE': 'YES',
+      },
+    },
+  ],
+}