Enable the -Wundef warning for clang

webrtc/modules/audio_coding/test/TestStereo.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_coding/test/TestStereo.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/include/audio_coding_module_typedefs.h"
 #include "webrtc/modules/audio_coding/test/utility.h"
 #include "webrtc/system_wrappers/include/trace.h"
 #include "webrtc/test/testsupport/fileutils.h"
 
 namespace webrtc {
 
 // Class for simulating packet handling
@@ skipping 809 matching lines @@
     printf("%s -> ", send_codec->plname);
   }
   CodecInst receive_codec;
   acm_b_->ReceiveCodec(&receive_codec);
   if (test_mode_ != 0) {
     printf("%s\n", receive_codec.plname);
   }
 }
 
 }  // namespace webrtc