Enable the -Wundef warning for clang

webrtc/base/mod_ops_unittest.cc

 /*
  *  Copyright (c) 2016 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/base/mod_ops.h"
 
 namespace webrtc {
 class TestModOps : public ::testing::Test {
  protected:
   // Can't use std::numeric_limits<unsigned long>::max() since
   // MSVC doesn't support constexpr.
   static const unsigned long ulmax = ~0ul;  // NOLINT
 };
 
@@ skipping 109 matching lines @@
   for (uint16_t i = 0; i < D / 2; ++i)
     ASSERT_EQ(i, (MinDiff<uint16_t, D>(0, i)));
 
   ASSERT_EQ(D / 2, (MinDiff<uint16_t, D>(0, D / 2)));
 
   for (uint16_t i = 0; i < D / 2; ++i)
     ASSERT_EQ(D / 2 - i, (MinDiff<uint16_t, D>(0, D / 2 - i)));
 }
 
 }  // namespace webrtc