Enable the -Wundef warning for clang

webrtc/modules/media_file/media_file_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/media_file/media_file.h"
 #include "webrtc/system_wrappers/include/sleep.h"
 #include "webrtc/test/testsupport/fileutils.h"
 
 class MediaFileTest : public testing::Test {
  protected:
   void SetUp() {
     // Use number 0 as the the identifier and pass to CreateMediaFile.
     media_file_ = webrtc::MediaFile::CreateMediaFile(0);
     ASSERT_TRUE(media_file_ != NULL);
@@ skipping 75 matching lines @@
   uint8_t header[kHeaderSize];
   ASSERT_EQ(1u, fread(header, kHeaderSize, 1, f));
   EXPECT_EQ(0, memcmp(kExpectedHeader, header, kHeaderSize));
 
   uint8_t payload[kPayloadSize];
   ASSERT_EQ(1u, fread(payload, kPayloadSize, 1, f));
   EXPECT_EQ(0, memcmp(kFakeData, payload, kPayloadSize));
 
   EXPECT_EQ(0, fclose(f));
 }