Enable the -Wundef warning for clang

webrtc/common_audio/real_fourier_unittest.cc

 /*
  *  Copyright (c) 2014 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/common_audio/real_fourier.h"
 
 #include <stdlib.h>
 
-#include "testing/gtest/include/gtest/gtest.h"
+#include "webrtc/test/gtest.h"
 #include "webrtc/common_audio/real_fourier_openmax.h"
 #include "webrtc/common_audio/real_fourier_ooura.h"
 
 namespace webrtc {
 
 using std::complex;
 
 TEST(RealFourierStaticsTest, AllocatorAlignment) {
   {
     RealFourier::fft_real_scoper real;
@@ skipping 74 matching lines @@
   this->rf_.Inverse(this->cplx_buffer_.get(), this->real_buffer_.get());
 
   EXPECT_NEAR(this->real_buffer_[0], 1.0f, 1e-8f);
   EXPECT_NEAR(this->real_buffer_[1], 2.0f, 1e-8f);
   EXPECT_NEAR(this->real_buffer_[2], 3.0f, 1e-8f);
   EXPECT_NEAR(this->real_buffer_[3], 4.0f, 1e-8f);
 }
 
 }  // namespace webrtc