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>