Enable the -Wundef warning for clang

webrtc/modules/audio_processing/splitting_filter_unittest.cc

 /*
  *  Copyright (c) 2014 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.
  */
 
 // MSVC++ requires this to be set before any other includes to get M_PI.
 #define _USE_MATH_DEFINES
 
 #include <cmath>
 
-#include "testing/gtest/include/gtest/gtest.h"
+#include "webrtc/test/gtest.h"
 #include "webrtc/common_audio/channel_buffer.h"
 #include "webrtc/modules/audio_processing/splitting_filter.h"
 
 namespace webrtc {
 namespace {
 
 const size_t kSamplesPer16kHzChannel = 160;
 const size_t kSamplesPer48kHzChannel = 480;
 
 }  // namespace
@@ skipping 70 matching lines @@
     for (size_t j = 0; j < kNumBands; ++j) {
       any_present |= is_present[j];
     }
     if (any_present) {
       EXPECT_GT(xcorr, kAmplitude * kAmplitude / 4);
     }
   }
 }
 
 }  // namespace webrtc