Enable the -Wundef warning for clang

webrtc/modules/rtp_rtcp/source/time_util_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 "webrtc/modules/rtp_rtcp/source/time_util.h"
 
-#include "testing/gtest/include/gtest/gtest.h"
+#include "webrtc/test/gtest.h"
 
 namespace webrtc {
 
 TEST(TimeUtilTest, CompactNtp) {
   const uint32_t kNtpSec = 0x12345678;
   const uint32_t kNtpFrac = 0x23456789;
   const NtpTime kNtp(kNtpSec, kNtpFrac);
   const uint32_t kNtpMid = 0x56782345;
   EXPECT_EQ(kNtpMid, CompactNtp(kNtp));
 }
@@ skipping 41 matching lines @@
   const NtpTime ntp1(0x20000, 0x23456);
   const NtpTime ntp2(0x1ffff, 0x64335);
   int64_t ms_diff = ntp2.ToMs() - ntp1.ToMs();
   ASSERT_GT(0, ms_diff);
   // Ntp difference close to 2^16 seconds should be treated as negative.
   uint32_t ntp_diff = CompactNtp(ntp2) - CompactNtp(ntp1);
   int64_t ntp_to_ms_diff = CompactNtpRttToMs(ntp_diff);
   EXPECT_EQ(1, ntp_to_ms_diff);
 }
 }  // namespace webrtc