Enable the -Wundef warning for clang

webrtc/system_wrappers/source/critical_section_unittest.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 "webrtc/system_wrappers/include/critical_section_wrapper.h"
 
-#include "testing/gtest/include/gtest/gtest.h"
+#include "webrtc/test/gtest.h"
 #include "webrtc/system_wrappers/include/sleep.h"
 #include "webrtc/base/platform_thread.h"
 #include "webrtc/system_wrappers/include/trace.h"
 
 namespace webrtc {
 
 namespace {
 
 // Cause a process switch. Needed to avoid depending on
 // busy-wait in tests.
@@ skipping 104 matching lines @@
 
   SwitchProcess();
   EXPECT_TRUE(WaitForCount(count_before + 1, &count));
   thread.Stop();
   delete crit_sect;
 }
 
 }  // anonymous namespace
 
 }  // namespace webrtc