Fix vm VS2013 compilation issue with c99_support_win.h.

runtime/platform/c99_support_win.h

 // Copyright (c) 2011, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #ifndef PLATFORM_C99_SUPPORT_WIN_H_
 #define PLATFORM_C99_SUPPORT_WIN_H_
 
-// Visual C++ is missing a bunch of C99 math macros and
-// functions. Define them here.
+#if defined(_MSC_VER) && (_MSC_VER < 1800)
+
+// Before Visual Studio 2013 Visual C++ was missing a bunch of C99 math macros
+// and functions. Define them here.
 
 #include <float.h>
 #include <string.h>
 
 static const unsigned __int64 kQuietNaNMask =
     static_cast<unsigned __int64>(0xfff) << 51;
 
 
 #ifndef va_copy
 #define va_copy(dst, src) (memmove(&(dst), &(src), sizeof(dst)))
@@ skipping 43 matching lines @@
   } else {
     return intpart - 1;
   }
 }
 
 // Windows does not have strtoll defined.
 #if defined(_MSC_VER)
 #define strtoll _strtoi64
 #endif
 
+#endif
 
 #endif  // PLATFORM_C99_SUPPORT_WIN_H_