Enable the -Wundef warning for clang

webrtc/modules/audio_coding/audio_network_adaptor/smoothing_filter_unittest.cc

 /*
  *  Copyright (c) 2016 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 <memory>
 
-#include "testing/gtest/include/gtest/gtest.h"
+#include "webrtc/test/gtest.h"
 #include "webrtc/modules/audio_coding/audio_network_adaptor/smoothing_filter.h"
 
 namespace webrtc {
 
 namespace {
 
 constexpr int kTimeConstantMs = 1000;
 constexpr float kMaxAbsError = 0.0001f;
 constexpr int64_t kClockInitialTime = 123456;
 
@@ skipping 75 matching lines @@
   constexpr int kTimeIntervalMs = 100;
   auto states = CreateSmoothingFilter();
   states.smoothing_filter->AddSample(0.0);
   states.simulated_clock->AdvanceTimeMilliseconds(kTimeConstantMs);
   CheckOutput(&states, kTimeIntervalMs, 1.0, 0.66713f);
   CheckOutput(&states, kTimeIntervalMs, 0.0, 0.60364f);
   CheckOutput(&states, kTimeIntervalMs, 1.0, 0.64136f);
 }
 
 }  // namespace webrtc