Enable the -Wundef warning for clang

webrtc/common_audio/audio_ring_buffer_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 <memory>
 
 #include "webrtc/common_audio/audio_ring_buffer.h"
 
-#include "testing/gtest/include/gtest/gtest.h"
+#include "webrtc/test/gtest.h"
 #include "webrtc/common_audio/channel_buffer.h"
 
 namespace webrtc {
 
 class AudioRingBufferTest :
     public ::testing::TestWithParam< ::testing::tuple<int, int, int, int> > {
 };
 
 void ReadAndWriteTest(const ChannelBuffer<float>& input,
                       size_t num_write_chunk_frames,
@@ skipping 77 matching lines @@
   buf.MoveReadPositionForward(3);
   ChannelBuffer<float> output(1, kNumChannels);
   buf.Read(output.channels(), kNumChannels, 1);
   EXPECT_EQ(4, output.channels()[0][0]);
   buf.MoveReadPositionBackward(3);
   buf.Read(output.channels(), kNumChannels, 1);
   EXPECT_EQ(2, output.channels()[0][0]);
 }
 
 }  // namespace webrtc