Enable the -Wundef warning for clang

webrtc/voice_engine/test/auto_test/voe_output_test.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 "testing/gtest/include/gtest/gtest.h"
+#include "webrtc/test/gtest.h"
 #include "webrtc/base/random.h"
 #include "webrtc/base/timeutils.h"
 #include "webrtc/system_wrappers/include/sleep.h"
 #include "webrtc/voice_engine/test/channel_transport/channel_transport.h"
 #include "webrtc/test/testsupport/fileutils.h"
 #include "webrtc/voice_engine/test/auto_test/voe_standard_test.h"
 
 namespace {
 
 const char kIp[] = "127.0.0.1";
@@ skipping 171 matching lines @@
     test.EnableOutputCheck();
     webrtc::SleepMs(random.Rand(kCheckTimeMs - kCheckTimeMs / 10,
                                 kCheckTimeMs + kCheckTimeMs / 10));
     test.DisableOutputCheck();
     test.SetBitRate(random.Rand(kMinOpusRate, kMaxOpusRate));
     test.Unmute();
   }
 }
 
 }  // namespace voetest