Enable the -Wundef warning for clang

webrtc/modules/audio_processing/beamformer/matrix_test_helpers.h

 /*
  *  Copyright (c) 2014 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.
  */
 
 #ifndef WEBRTC_MODULES_AUDIO_PROCESSING_BEAMFORMER_MATRIX_TEST_HELPERS_H_
 #define WEBRTC_MODULES_AUDIO_PROCESSING_BEAMFORMER_MATRIX_TEST_HELPERS_H_
 
-#include "testing/gtest/include/gtest/gtest.h"
+#include "webrtc/test/gtest.h"
 #include "webrtc/modules/audio_processing/beamformer/complex_matrix.h"
 #include "webrtc/modules/audio_processing/beamformer/matrix.h"
 
 namespace {
 const float kTolerance = 0.001f;
 }
 
 namespace webrtc {
 
 using std::complex;
@@ skipping 68 matching lines @@
                     actual_elements[i][j].imag(),
                     tolerance);
       }
     }
   }
 };
 
 }  // namespace webrtc
 
 #endif  // WEBRTC_MODULES_AUDIO_PROCESSING_BEAMFORMER_MATRIX_TEST_HELPERS_H_