Enable the -Wundef warning for clang

webrtc/modules/audio_coding/codecs/isac/fix/source/filters_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.
  */
-#include "testing/gtest/include/gtest/gtest.h"
+#include "webrtc/test/gtest.h"
 #include "webrtc/modules/audio_coding/codecs/isac/fix/source/codec.h"
 #include "webrtc/system_wrappers/include/cpu_features_wrapper.h"
 #include "webrtc/typedefs.h"
 
 class FiltersTest : public testing::Test {
  protected:
   // Pass a function pointer to the Tester function.
   void FiltersTester(AutocorrFix WebRtcIsacfix_AutocorrFixFunction) {
     const int kOrder = 12;
     const int kBuffer = 40;
@@ skipping 35 matching lines @@
     EXPECT_EQ(0, scale);
   }
 };
 
 TEST_F(FiltersTest, AutocorrFixTest) {
   FiltersTester(WebRtcIsacfix_AutocorrC);
 #if defined(WEBRTC_HAS_NEON)
   FiltersTester(WebRtcIsacfix_AutocorrNeon);
 #endif
 }