Enable the -Wundef warning for clang

webrtc/modules/audio_coding/codecs/isac/main/source/audio_encoder_isac_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 <limits>
 
-#include "testing/gtest/include/gtest/gtest.h"
+#include "webrtc/test/gtest.h"
 #include "webrtc/modules/audio_coding/codecs/isac/main/include/audio_encoder_isac.h"
 
 namespace webrtc {
 
 namespace {
 
 void TestBadConfig(const AudioEncoderIsac::Config& config) {
   EXPECT_FALSE(config.IsOk());
 }
 
@@ skipping 23 matching lines @@
   // Try some unreasonable values and watch them fail.
   config.bit_rate = -1;
   S(TestBadConfig(config));
   config.bit_rate = 1;
   S(TestBadConfig(config));
   config.bit_rate = std::numeric_limits<int>::max();
   S(TestBadConfig(config));
 }
 
 }  // namespace webrtc