Enable the -Wundef warning for clang

webrtc/modules/audio_conference_mixer/source/memory_pool.h

 /*
  *  Copyright (c) 2011 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.
  */
 
 #ifndef WEBRTC_MODULES_AUDIO_CONFERENCE_MIXER_SOURCE_MEMORY_POOL_H_
 #define WEBRTC_MODULES_AUDIO_CONFERENCE_MIXER_SOURCE_MEMORY_POOL_H_
 
 #include <assert.h>
 
 #include "webrtc/typedefs.h"
 
-#if _WIN32
+#ifdef _WIN32
 #include "webrtc/modules/audio_conference_mixer/source/memory_pool_win.h"
 #else
 #include "webrtc/modules/audio_conference_mixer/source/memory_pool_posix.h"
 #endif
 
 namespace webrtc {
 
 template<class MemoryType>
 class MemoryPool
 {
@@ skipping 84 matching lines @@
 {
     if(memory == NULL)
     {
         return -1;
     }
     return _ptrImpl->PushMemory(memory);
 }
 }  // namespace webrtc
 
 #endif // WEBRTC_MODULES_AUDIO_CONFERENCE_MIXER_SOURCE_MEMORY_POOL_H_