Enable the -Wundef warning for clang

webrtc/modules/audio_processing/vad/pitch_based_vad_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/modules/audio_processing/vad/pitch_based_vad.h"
 
 #include <math.h>
 #include <stdio.h>
 
 #include <string>
 
-#include "testing/gtest/include/gtest/gtest.h"
+#include "webrtc/test/gtest.h"
 #include "webrtc/test/testsupport/fileutils.h"
 
 namespace webrtc {
 
 TEST(PitchBasedVadTest, VoicingProbabilityTest) {
   std::string spectral_peak_file_name =
       test::ResourcePath("audio_processing/agc/agc_spectral_peak", "dat");
   FILE* spectral_peak_file = fopen(spectral_peak_file_name.c_str(), "rb");
   ASSERT_TRUE(spectral_peak_file != NULL);
 
@@ skipping 37 matching lines @@
     EXPECT_EQ(0, vad_.VoicingProbability(audio_features, &p));
     EXPECT_NEAR(p, reference_activity_probability, 0.01);
   }
 
   fclose(spectral_peak_file);
   fclose(pitch_gain_file);
   fclose(pitch_lag_file);
 }
 
 }  // namespace webrtc