Enable the -Wundef warning for clang

webrtc/system_wrappers/source/event_timer_posix_unittest.cc

 /*
  *  Copyright (c) 2016 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/source/event_timer_posix.h"
 
-#include "testing/gtest/include/gtest/gtest.h"
+#include "webrtc/test/gtest.h"
 #include "webrtc/base/event.h"
 #include "webrtc/base/criticalsection.h"
 
 namespace webrtc {
 
 enum class ThreadState {
   kNotStarted,
   kWaiting,
   kRequestProcessCall,
   kCallingProcess,
@@ skipping 165 matching lines @@
   ASSERT_TRUE(StartTimer(false, kTimerIntervalMs));
   ASSERT_TRUE(CallProcess(kTimeoutMs));
 
   EventTypeWrapper res = Wait(kTimeoutMs);
   EXPECT_EQ(kEventSignaled, res);
 
   ASSERT_TRUE(AwaitProcessDone(kTimeoutMs));
 }
 
 }  // namespace webrtc