Have Clang builders build in C++11 mode.

gyp/libwebp.gyp

 # Copyright (c) 2012 The Chromium Authors. All rights reserved.
 # 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,
     'conditions':[
       ['skia_android_framework == 1', {
         'use_system_libwebp': 1,
       }, {
         'use_system_libwebp%': 0,
       }],
     ],
   },
   'conditions': [
     ['use_system_libwebp==0', {
@@ skipping 135 matching lines @@
             'libwebp_demux',
             'libwebp_dsp',
             'libwebp_dsp_neon',
             'libwebp_enc',
             'libwebp_utils',
           ],
           'direct_dependent_settings': {
             'include_dirs': [
               '../third_party/externals/libwebp/src',
             ],
+            'cflags': [
+              '-Wno-unused-function',  # In C++11 mode, we get this warning when including decode.h.
+            ]
           },
           'conditions': [
             ['OS!="win"', {'product_name': 'webp'}],
           ],
         },
       ],
     }, {
       # use_system_libwep == 1
       'targets': [
         {
@@ skipping 22 matching lines @@
                   ],
                 },
               },
             ],
           ],
         }
       ],
     }],
   ],
 }