configure: Enable MinGW-w64's implementation of vsnprintf and pals

As of r5799 in MinGW-w64's trunk, which is he upcoming major version 3,
the POSIX-compliant vsnprintf implementation is disabled by default.
Enable it for this version. Keep using out own implementation for
prior versions, because it is broken in them.

Relevant Reference Thread: http://article.gmane.org/gmane.comp.gnu.mingw.w64.general/7133

Signed-off-by: Derek Buitenhuis <derek.buitenhuis@gmail.com>
avisynth: Make sure the filename passed to Avisynth is in the right code page

Avisynth is a non-unicode application and cannot accept UTF-8
characters. Therefore, the input filename should be converted to
the code page that it expects.

Signed-off-by: Derek Buitenhuis <derek.buitenhuis@gmail.com>
sonic: use av_calloc()

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
sonic: simplify quant cliping

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
sonic: avoid float sqrt() for integer input & output

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>