Enable the -Wundef warning for clang

webrtc/modules/audio_coding/test/iSACTest.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/modules/audio_coding/test/iSACTest.h"
 
 #include <ctype.h>
 #include <stdio.h>
 #include <string.h>
 
-#if _WIN32
+#ifdef _WIN32
 #include <windows.h>
-#elif WEBRTC_LINUX
+#elif defined(WEBRTC_LINUX)
 #include <time.h>
 #else
 #include <sys/time.h>
 #include <time.h>
 #endif
 
 #include "webrtc/modules/audio_coding/acm2/acm_common_defs.h"
 #include "webrtc/modules/audio_coding/test/utility.h"
 #include "webrtc/system_wrappers/include/event_wrapper.h"
 #include "webrtc/system_wrappers/include/trace.h"
@@ skipping 306 matching lines @@
       numSendCodecChanged++;
     }
   }
   _outFileA.Close();
   _outFileB.Close();
   _inFileA.Close();
   _inFileB.Close();
 }
 
 }  // namespace webrtc