Enable the -Wundef warning for clang

webrtc/system_wrappers/source/logging_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/logging.h"
 
-#include "testing/gtest/include/gtest/gtest.h"
+#include "webrtc/test/gtest.h"
 #include "webrtc/base/arraysize.h"
 #include "webrtc/base/event.h"
 #include "webrtc/system_wrappers/include/trace.h"
 
 namespace webrtc {
 namespace {
 const char kTestLogString[] = "Incredibly important test message!(?)";
 const int kTestLevel = kTraceWarning;
 
 class LoggingTestCallback : public TraceCallback {
@@ skipping 24 matching lines @@
   LoggingTestCallback callback(&event);
   Trace::SetTraceCallback(&callback);
 
   LOG(LS_WARNING) << kTestLogString;
   EXPECT_TRUE(event.Wait(2000));
 
   Trace::SetTraceCallback(nullptr);
   Trace::ReturnTrace();
 }
 }  // namespace webrtc