Enable the -Wundef warning for clang

webrtc/modules/audio_processing/aec/system_delay_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 "testing/gtest/include/gtest/gtest.h"
+#include "webrtc/test/gtest.h"
 #include "webrtc/modules/audio_processing/aec/aec_core.h"
 #include "webrtc/modules/audio_processing/aec/echo_cancellation.h"
 #include "webrtc/typedefs.h"
 namespace webrtc {
 namespace {
 
 class SystemDelayTest : public ::testing::Test {
  protected:
   SystemDelayTest();
   virtual void SetUp();
@@ skipping 568 matching lines @@
         }
         // Verify we are not in a non-causal state.
         EXPECT_FALSE(non_causal);
       }
     }
   }
 }
 
 }  // namespace
 }  // namespace webrtc