Enable the -Wundef warning for clang

webrtc/voice_engine/test/auto_test/automated_mode.cc

 /*
  *  Copyright (c) 2011 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/test/testsupport/fileutils.h"
 
 void InitializeGoogleTest(int* argc, char** argv) {
   // Initialize WebRTC testing framework so paths to resources can be resolved.
   webrtc::test::SetExecutablePath(argv[0]);
   testing::InitGoogleTest(argc, argv);
 }
 
 int RunInAutomatedMode() {
   return RUN_ALL_TESTS();
 }