Enable the -Wundef warning for clang

webrtc/modules/audio_coding/test/opus_test.cc

 /*
  *  Copyright (c) 2013 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_coding/test/opus_test.h"
 
 #include <assert.h>
 
 #include <string>
 
-#include "testing/gtest/include/gtest/gtest.h"
+#include "webrtc/test/gtest.h"
 #include "webrtc/common_types.h"
 #include "webrtc/engine_configurations.h"
 #include "webrtc/modules/audio_coding/codecs/opus/opus_interface.h"
 #include "webrtc/modules/audio_coding/include/audio_coding_module_typedefs.h"
 #include "webrtc/modules/audio_coding/test/TestStereo.h"
 #include "webrtc/modules/audio_coding/test/utility.h"
 #include "webrtc/system_wrappers/include/trace.h"
 #include "webrtc/test/testsupport/fileutils.h"
 
 namespace webrtc {
@@ skipping 349 matching lines @@
   out_file_.Open(file_name, 48000, "wb");
   file_stream.str("");
   file_name = file_stream.str();
   file_stream << webrtc::test::OutputPath() << "opusstandalone_out_"
       << test_number << ".pcm";
   file_name = file_stream.str();
   out_file_standalone_.Open(file_name, 48000, "wb");
 }
 
 }  // namespace webrtc