set __USE_MINGW_ANSI_STDIO=1 to let vsnprintf in mingw64 conform with ANSI

With this fix, T::M::Hoedown passes 02_toc.t under Win32 (tested with strawberry 64bit).

Related links:
- http://www.mail-archive.com/mingw-w64-public@lists.sourceforge.net/msg08018.html
- http://git.videolan.org/?p=ffmpeg.git;a=commitdiff;h=8bdbabfaa46989b1a0e82d2df0a27df32277294d