Enable the -Wundef warning for clang

webrtc/modules/audio_processing/intelligibility/intelligibility_utils_unittest.cc

 /*
  *  Copyright (c) 2015 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 <cmath>
 #include <complex>
 #include <vector>
 
-#include "testing/gtest/include/gtest/gtest.h"
+#include "webrtc/test/gtest.h"
 #include "webrtc/base/arraysize.h"
 #include "webrtc/modules/audio_processing/intelligibility/intelligibility_utils.h"
 
 namespace webrtc {
 
 namespace intelligibility {
 
 std::vector<std::vector<std::complex<float>>> GenerateTestData(size_t freqs,
                                                                size_t samples) {
   std::vector<std::vector<std::complex<float>>> data(samples);
@@ skipping 44 matching lines @@
       EXPECT_LT(out_data[i][j].real(), 1.f);
       EXPECT_GT(out_data[i][j].imag(), 0.f);
       EXPECT_LT(out_data[i][j].imag(), 1.f);
     }
   }
 }
 
 }  // namespace intelligibility
 
 }  // namespace webrtc