Enable the -Wundef warning for clang

webrtc/modules/audio_processing/utility/block_mean_calculator_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 "testing/gtest/include/gtest/gtest.h"
+#include "webrtc/test/gtest.h"
 
 #include "webrtc/modules/audio_processing/utility/block_mean_calculator.h"
 
 namespace webrtc {
 
 TEST(MeanCalculatorTest, Correctness) {
   const size_t kBlockLength = 10;
   BlockMeanCalculator mean_calculator(kBlockLength);
   size_t i = 0;
   float reference = 0.0;
@@ skipping 28 matching lines @@
   for (; i < kBlockLength - 1; ++i) {
     mean_calculator.AddValue(static_cast<float>(i));
     EXPECT_FALSE(mean_calculator.EndOfBlock());
   }
   mean_calculator.AddValue(static_cast<float>(i));
   EXPECT_TRUE(mean_calculator.EndOfBlock());
   EXPECT_EQ(mean_calculator.GetLatestMean(), 0.5 * (kBlockLength - 1));
 }
 
 }  // namespace webrtc