MinGW64 has this prototype already.

/* The MinGW headers are missing a few Win32 function definitions,

   you shouldn't need this if you use VC++ */

#ifdef __MINGW32__

#if defined(__MINGW32__) && !defined(__MINGW64__)

int __cdecl _snscanf(const char * input, size_t length, const char * format, ...);

#endif