Enable the -Wundef warning for clang

webrtc/system_wrappers/source/ntp_time_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 "testing/gtest/include/gtest/gtest.h"
+#include "webrtc/test/gtest.h"
 
 #include "webrtc/system_wrappers/include/ntp_time.h"
 
 namespace webrtc {
 namespace {
 
 const uint32_t kNtpSec = 0x12345678;
 const uint32_t kNtpFrac = 0x23456789;
 
 TEST(NtpTimeTest, NoValueMeansInvalid) {
@@ skipping 38 matching lines @@
 TEST(NtpTimeTest, ToMsMeansToNtpMilliseconds) {
   SimulatedClock clock(0x123456789abc);
 
   NtpTime ntp(clock);
   EXPECT_EQ(ntp.ToMs(), Clock::NtpToMs(ntp.seconds(), ntp.fractions()));
   EXPECT_EQ(ntp.ToMs(), clock.CurrentNtpInMilliseconds());
 }
 
 }  // namespace
 }  // namespace webrtc