Silence giflib warning on mac

gyp/giflib.gyp

 # Copyright 2014 Google Inc.
 #
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 {
   'variables': {
     'skia_warnings_as_errors': 0,
   },
   'targets': [
@@ skipping 12 matching lines @@
             'dependencies': [
             ],
             'direct_dependent_settings': {
               'include_dirs': [
                 '../third_party/externals/giflib',
               ],
             },
             'cflags': [
               '-w',
             ],
+            'xcode_settings': {
+              'WARNING_CFLAGS': [
+                '-w'
+              ],
+            },
             'sources': [
               '../third_party/externals/giflib/dgif_lib.c',
               '../third_party/externals/giflib/gifalloc.c',
               '../third_party/externals/giflib/gif_err.c',
             ],
           }, { # skia_android_framework
             'type': 'none',
             'direct_dependent_settings': {
               'libraries' : [
                 'libgif.a',
               ],
               'include_dirs': [
                 'external/giflib',
               ]
             }
           }
         ]
       ]
     }
   ]
 }