Enable the -Wundef warning for clang

webrtc/modules/audio_processing/transient/wpd_tree_unittest.cc

 /*
  *  Copyright (c) 2013 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/modules/audio_processing/transient/wpd_tree.h"
 
 #include <memory>
 #include <sstream>
 #include <string>
 
-#include "testing/gtest/include/gtest/gtest.h"
+#include "webrtc/test/gtest.h"
 #include "webrtc/modules/audio_processing/transient/daubechies_8_wavelet_coeffs.h"
 #include "webrtc/modules/audio_processing/transient/file_utils.h"
 #include "webrtc/system_wrappers/include/file_wrapper.h"
 #include "webrtc/test/testsupport/fileutils.h"
 
 namespace webrtc {
 
 TEST(WPDTreeTest, Construction) {
   const size_t kTestBufferSize = 100;
   const int kLevels = 5;
@@ skipping 156 matching lines @@
   // Close all matlab and out files.
   for (int i = 0; i < kLeaves; ++i) {
     matlab_files_data[i]->CloseFile();
     out_files_data[i]->CloseFile();
   }
 
   test_file->CloseFile();
 }
 
 }  // namespace webrtc