Enable the -Wundef warning for clang

webrtc/base/pathutils.cc

 /*
  *  Copyright 2004 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.
  */
 
@@ skipping 15 matching lines @@
 
 static const char EMPTY_STR[] = "";
 
 // EXT_DELIM separates a file basename from extension
 const char EXT_DELIM = '.';
 
 // FOLDER_DELIMS separate folder segments and the filename
 const char* const FOLDER_DELIMS = "/\\";
 
 // DEFAULT_FOLDER_DELIM is the preferred delimiter for this platform
-#if WEBRTC_WIN
+#ifdef WEBRTC_WIN
 const char DEFAULT_FOLDER_DELIM = '\\';
 #else  // !WEBRTC_WIN
 const char DEFAULT_FOLDER_DELIM = '/';
 #endif  // !WEBRTC_WIN
 
 ///////////////////////////////////////////////////////////////////////////////
 // Pathname - parsing of pathnames into components, and vice versa
 ///////////////////////////////////////////////////////////////////////////////
 
 bool Pathname::IsFolderDelimiter(char ch) {
@@ skipping 202 matching lines @@
   // sanity checking
   return (isalpha(drive[0]) &&
           drive[1] == ':' &&
           drive[2] == '\\');
 }
 #endif
 
 ///////////////////////////////////////////////////////////////////////////////
 
 } // namespace rtc