Enable the -Wundef warning for clang

webrtc/modules/audio_processing/agc/loudness_histogram_unittest.cc

 /*
  *  Copyright (c) 2012 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.
  */
 
 // Use CreateHistUnittestFile.m to generate the input file.
 
 #include "webrtc/modules/audio_processing/agc/loudness_histogram.h"
 
 #include <stdio.h>
 #include <algorithm>
 #include <cmath>
 #include <memory>
 
-#include "gtest/gtest.h"
+#include "webrtc/test/gtest.h"
 #include "webrtc/test/testsupport/fileutils.h"
 #include "webrtc/modules/audio_processing/agc/utility.h"
 
 namespace webrtc {
 
 struct InputOutput {
   double rms;
   double activity_probability;
   double audio_content;
   double loudness;
@@ skipping 66 matching lines @@
                     .c_str());
 }
 
 TEST_F(LoudnessHistogramTest, InactiveCircularBuffer) {
   RunTest(false, test::ResourcePath(
                      "audio_processing/agc/agc_no_circular_buffer", "dat")
                      .c_str());
 }
 
 }  // namespace webrtc