Enable the -Wundef warning for clang

webrtc/base/platform_thread_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/base/platform_thread.h"
 
-#include "testing/gtest/include/gtest/gtest.h"
+#include "webrtc/test/gtest.h"
 #include "webrtc/system_wrappers/include/sleep.h"
 
 namespace rtc {
 namespace {
 // Function that does nothing, and reports success.
 bool NullRunFunction(void* obj) {
   webrtc::SleepMs(0);  // Hand over timeslice, prevents busy looping.
   return true;
 }
 
@@ skipping 32 matching lines @@
   PlatformThread thread(&SetFlagRunFunction, &flag, "RunFunctionIsCalled");
   thread.Start();
 
   // At this point, the flag may be either true or false.
   thread.Stop();
 
   // We expect the thread to have run at least once.
   EXPECT_TRUE(flag);
 }
 }  // rtc