Enable the -Wundef warning for clang

webrtc/modules/audio_processing/config_unittest.cc

 /*
  *  Copyright (c) 2013 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 "webrtc/modules/audio_processing/include/config.h"
 
-#include "testing/gtest/include/gtest/gtest.h"
+#include "webrtc/test/gtest.h"
 
 namespace webrtc {
 namespace {
 
 struct MyExperiment {
   static const ConfigOptionID identifier = ConfigOptionID::kMyExperimentForTest;
   static const int kDefaultFactor;
   static const int kDefaultOffset;
 
   MyExperiment()
@@ skipping 51 matching lines @@
   }
 };
 
 TEST(Config, SupportsPolymorphism) {
   Config config;
   config.Set<Algo1_CostFunction>(new SqrCost());
   EXPECT_EQ(25, config.Get<Algo1_CostFunction>().cost(5));
 }
 }  // namespace
 }  // namespace webrtc