Enable the -Wundef warning for clang

webrtc/base/httpcommon.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 370 matching lines @@
     if (!find_string(zindex, zone, kTimeZones, arraysize(kTimeZones))) {
       return false;
     }
     gmt = non_gmt + kTimeZoneOffsets[zindex] * 60 * 60;
   }
   // TODO: Android should support timezone, see b/2441195
 #if defined(WEBRTC_MAC) && !defined(WEBRTC_IOS) || defined(WEBRTC_ANDROID) || defined(BSD)
   tm *tm_for_timezone = localtime(&gmt);
   *seconds = gmt + tm_for_timezone->tm_gmtoff;
 #else
-#if _MSC_VER >= 1900
+#if defined(_MSC_VER) && _MSC_VER >= 1900
   long timezone = 0;
   _get_timezone(&timezone);
 #endif
   *seconds = gmt - timezone;
 #endif
   return true;
 }
 
 std::string HttpAddress(const SocketAddress& address, bool secure) {
   return (address.port() == HttpDefaultPort(secure))
@@ skipping 644 matching lines @@
   }
 #endif
 #endif // WEBRTC_WIN
 
   return HAR_IGNORE;
 }
 
 //////////////////////////////////////////////////////////////////////
 
 } // namespace rtc