Enable the -Wundef warning for clang

webrtc/test/fake_decoder.cc

 /*
  *  Copyright (c) 2013 The WebRTC project authors. All Rights Reserved.
  *
  *  Use of this source code is governed by a BSD-style license
  *  that can be found in the LICENSE file in the root of the source
  *  tree. An additional intellectual property rights grant can be found
  *  in the file PATENTS.  All contributing project authors may
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
 #include "webrtc/test/fake_decoder.h"
 
-#include "testing/gtest/include/gtest/gtest.h"
+#include "webrtc/test/gtest.h"
 
 namespace webrtc {
 namespace test {
 
 FakeDecoder::FakeDecoder() : callback_(NULL) {}
 
 int32_t FakeDecoder::InitDecode(const VideoCodec* config,
                                 int32_t number_of_cores) {
   config_ = *config;
   size_t width = config->width;
@@ skipping 56 matching lines @@
   }
   return FakeDecoder::Decode(input,
                              missing_frames,
                              fragmentation,
                              codec_specific_info,
                              render_time_ms);
 }
 
 }  // namespace test
 }  // namespace webrtc