Enable the -Wundef warning for clang

webrtc/test/testsupport/fileutils_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 "webrtc/test/testsupport/fileutils.h"
 
 #include <stdio.h>
 
 #include <list>
 #include <string>
 
-#include "testing/gtest/include/gtest/gtest.h"
+#include "webrtc/test/gtest.h"
 
 #ifdef WIN32
 #define chdir _chdir
 static const char* kPathDelimiter = "\\";
 #else
 static const char* kPathDelimiter = "/";
 #endif
 
 static const std::string kResourcesDir = "resources";
 static const std::string kTestName = "fileutils_unittest";
@@ skipping 115 matching lines @@
   fclose(file);
   ASSERT_GT(webrtc::test::GetFileSize(std::string(temp_filename.c_str())), 0u);
   remove(temp_filename.c_str());
 }
 
 TEST_F(FileUtilsTest, GetFileSizeNonExistingFile) {
   ASSERT_EQ(0u, webrtc::test::GetFileSize("non-existing-file.tmp"));
 }
 
 }  // namespace webrtc