mac ninja&make: Expose SDKROOT to the environment.

test/mac/sdkroot/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.
 {
   'targets': [
     {
       'target_name': 'mytarget',
       'type': 'executable',
       'sources': [ 'file.cc', ],
       'xcode_settings': {
         'SDKROOT': 'macosx10.6',
       },
+      'postbuilds': [
+        {
+          'postbuild_name': 'envtest',
+          'action': [ './test_shorthand.sh', ],
+        },
+      ],
     },
   ],
 }