Enable the -Wundef warning for clang

webrtc/modules/audio_coding/test/Tester.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 <stdio.h>
 #include <string>
 #include <vector>
 
-#include "testing/gtest/include/gtest/gtest.h"
+#include "webrtc/test/gtest.h"
 #include "webrtc/modules/audio_coding/include/audio_coding_module.h"
 #include "webrtc/modules/audio_coding/test/APITest.h"
 #include "webrtc/modules/audio_coding/test/EncodeDecodeTest.h"
 #include "webrtc/modules/audio_coding/test/iSACTest.h"
 #include "webrtc/modules/audio_coding/test/opus_test.h"
 #include "webrtc/modules/audio_coding/test/PacketLossTest.h"
 #include "webrtc/modules/audio_coding/test/TestAllCodecs.h"
 #include "webrtc/modules/audio_coding/test/TestRedFec.h"
 #include "webrtc/modules/audio_coding/test/TestStereo.h"
 #include "webrtc/modules/audio_coding/test/TestVADDTX.h"
@@ skipping 146 matching lines @@
 // for offline testing. User interaction is needed.
 #ifdef ACM_TEST_FULL_API
   TEST(AudioCodingModuleTest, TestAPI) {
     Trace::CreateTrace();
     Trace::SetTraceFile((webrtc::test::OutputPath() +
         "acm_apitest_trace.txt").c_str());
     webrtc::APITest().Perform();
     Trace::ReturnTrace();
   }
 #endif