Enable the -Wundef warning for clang

webrtc/modules/audio_coding/acm2/codec_manager_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 "testing/gtest/include/gtest/gtest.h"
+#include "webrtc/test/gtest.h"
 #include "webrtc/modules/audio_coding/codecs/mock/mock_audio_encoder.h"
 #include "webrtc/modules/audio_coding/acm2/codec_manager.h"
 #include "webrtc/modules/audio_coding/acm2/rent_a_codec.h"
 
 namespace webrtc {
 namespace acm2 {
 
 using ::testing::Return;
 
 namespace {
@@ skipping 38 matching lines @@
   EXPECT_TRUE(cm.GetStackParams()->use_codec_fec);
 
   // Switch to a codec that doesn't support it.
   cm.GetStackParams()->speech_encoder = std::move(enc2);
   EXPECT_TRUE(rac.RentEncoderStack(cm.GetStackParams()));
   EXPECT_FALSE(cm.GetStackParams()->use_codec_fec);
 }
 
 }  // namespace acm2
 }  // namespace webrtc