Enable the -Wundef warning for clang

webrtc/base/common.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.
  */
 
 #include <signal.h>
 #include <stdlib.h>
 #include <stdio.h>
 #include <string.h>
 
-#if WEBRTC_WIN
+#ifdef WEBRTC_WIN
 #define WIN32_LEAN_AND_MEAN
 #include <windows.h>
 #endif  // WEBRTC_WIN 
 
 #if defined(WEBRTC_MAC) && !defined(WEBRTC_IOS)
 #include <CoreServices/CoreServices.h>
 #endif  // WEBRTC_MAC && !defined(WEBRTC_IOS)
 
 #include <algorithm>
 #include "webrtc/base/common.h"
 #include "webrtc/base/logging.h"
 
 //////////////////////////////////////////////////////////////////////
 // Assertions
 //////////////////////////////////////////////////////////////////////
 
 namespace rtc {
 
 void Break() {
-#if WEBRTC_WIN
+#ifdef WEBRTC_WIN
   ::DebugBreak();
 #else  // !WEBRTC_WIN 
   // On POSIX systems, SIGTRAP signals debuggers to break without killing the
   // process. If a debugger isn't attached, the uncaught SIGTRAP will crash the
   // app.
   raise(SIGTRAP);
 #endif
   // If a debugger wasn't attached, we will have crashed by this point. If a
   // debugger is attached, we'll continue from here.
 }
@@ skipping 16 matching lines @@
 
 bool IsOdd(int n) {
   return (n & 0x1);
 }
 
 bool IsEven(int n) {
   return !IsOdd(n);
 }
 
 } // namespace rtc