On MinGW-w64, vsnprintf returns -1 when given buffer is small
(it should return required length as defined in C99).

Because of this, bufprintf sometime fails to write string to the given
buffer, and header HTML tag (such as <h1> and </h1>) isn't outputed
occasionally.

Fix it by defining __USE_MINGW_ANSI_STDIO.
(see https://github.com/oneclick/rubyinstaller/issues/186 for detail)

Fixes #474