Enable the -Wundef warning for clang

webrtc/voice_engine/transmit_mixer_unittest.cc

 /*
  *  Copyright (c) 2012 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/voice_engine/transmit_mixer.h"
 
-#include "testing/gtest/include/gtest/gtest.h"
+#include "webrtc/test/gtest.h"
 #include "webrtc/voice_engine/include/voe_external_media.h"
 
 namespace webrtc {
 namespace voe {
 namespace {
 
 class MediaCallback : public VoEMediaProcess {
  public:
   virtual void Process(int channel, ProcessingTypes type,
                        int16_t audio[], size_t samples_per_channel,
@@ skipping 26 matching lines @@
   EXPECT_EQ(-1, tm->DeRegisterExternalMediaProcessing(kRecordingPerChannel));
   EXPECT_EQ(0, tm->DeRegisterExternalMediaProcessing(
                    kRecordingAllChannelsMixed));
   EXPECT_EQ(0, tm->DeRegisterExternalMediaProcessing(kRecordingPreprocessing));
   TransmitMixer::Destroy(tm);
 }
 
 }  // namespace
 }  // namespace voe
 }  // namespace webrtc