Enable the -Wundef warning for clang

webrtc/modules/audio_processing/vad/voice_activity_detector_unittest.cc

 /*
  *  Copyright (c) 2015 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/modules/audio_processing/vad/voice_activity_detector.h"
 
 #include <algorithm>
 #include <vector>
 
-#include "testing/gtest/include/gtest/gtest.h"
+#include "webrtc/test/gtest.h"
 #include "webrtc/test/testsupport/fileutils.h"
 
 namespace webrtc {
 namespace {
 
 const int kStartTimeSec = 16;
 const float kMeanSpeechProbability = 0.3f;
 const float kMaxNoiseProbability = 0.1f;
 const size_t kNumChunks = 300u;
 const size_t kNumChunksPerIsacBlock = 3;
@@ skipping 132 matching lines @@
     // the default value, 1.f, which would ruin the |max_probability| value.
     if (i > kNumChunksPerIsacBlock) {
       max_probability = std::max(max_probability, vad.last_voice_probability());
     }
   }
 
   EXPECT_LT(max_probability, kMaxNoiseProbability);
 }
 
 }  // namespace webrtc