Enable the -Wundef warning for clang

webrtc/voice_engine/voe_base_unittest.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/voice_engine/include/voe_base.h"
 
-#include "testing/gtest/include/gtest/gtest.h"
+#include "webrtc/test/gtest.h"
 #include "webrtc/modules/audio_processing/include/audio_processing.h"
 #include "webrtc/voice_engine/channel_manager.h"
 #include "webrtc/voice_engine/shared_data.h"
 #include "webrtc/voice_engine/voice_engine_fixture.h"
 #include "webrtc/voice_engine/voice_engine_impl.h"
 
 namespace webrtc {
 
 class VoEBaseTest : public VoiceEngineFixture {};
 
@@ skipping 54 matching lines @@
   EXPECT_EQ(1, reference.use_count());
 }
 
 TEST_F(VoEBaseTest, GetVersion) {
   char v1[1024] = {75};
   base_->GetVersion(v1);
   std::string v2 = VoiceEngine::GetVersionString() + "\n";
   EXPECT_EQ(v2, v1);
 }
 }  // namespace webrtc