Get libvpx to compile on VS2013.

source/libvpx/vp9/common/vp9_systemdependent.h

 /*
  *  Copyright (c) 2010 The WebM 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 VP9_COMMON_VP9_SYSTEMDEPENDENT_H_
 #define VP9_COMMON_VP9_SYSTEMDEPENDENT_H_
 
 #ifdef _MSC_VER
 #include <math.h>
 #endif
 
 #include "./vpx_config.h"
 #if ARCH_X86 || ARCH_X86_64
 void vpx_reset_mmx_state(void);
 #define vp9_clear_system_state() vpx_reset_mmx_state()
 #else
 #define vp9_clear_system_state()
 #endif
 
-#ifdef _MSC_VER
-// round is not defined in MSVC
+#if defined(_MSC_VER) && _MSC_VER < 1800
+// round is not defined in MSVC before VS2013.
 static int round(double x) {
   if (x < 0)
     return (int)ceil(x - 0.5);
   else
     return (int)floor(x + 0.5);
 }
 #endif
 
 struct VP9Common;
 void vp9_machine_specific_config(struct VP9Common *cm);
 
 #endif  // VP9_COMMON_VP9_SYSTEMDEPENDENT_H_