Enable the -Wundef warning for clang

webrtc/modules/audio_coding/codecs/red/audio_encoder_copy_red_unittest.cc

 /*
  *  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.
  */
 
 #include <memory>
 #include <vector>
 
-#include "testing/gtest/include/gtest/gtest.h"
+#include "webrtc/test/gtest.h"
 #include "webrtc/base/checks.h"
 #include "webrtc/modules/audio_coding/codecs/red/audio_encoder_copy_red.h"
 #include "webrtc/modules/audio_coding/codecs/mock/mock_audio_encoder.h"
 
 using ::testing::Return;
 using ::testing::_;
 using ::testing::SetArgPointee;
 using ::testing::InSequence;
 using ::testing::Invoke;
 using ::testing::MockFunction;
@@ skipping 272 matching lines @@
   config.speech_encoder = NULL;
   EXPECT_DEATH(red = new AudioEncoderCopyRed(std::move(config)),
                "Speech encoder not provided.");
   // The delete operation is needed to avoid leak reports from memcheck.
   delete red;
 }
 
 #endif  // GTEST_HAS_DEATH_TEST && !defined(WEBRTC_ANDROID)
 
 }  // namespace webrtc