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/7133Signed-off-by: Derek Buitenhuis <derek.buitenhuis@gmail.com>

configure

@@ -3627,6 +3627,8 @@ elif check_header _mingw.h; then
             __MINGW64_VERSION_MAJOR < 3"; then
         add_compat msvcrt/snprintf.o
         add_cflags "-include $source_path/compat/msvcrt/snprintf.h"
+    else
+        add_cppflags -D__USE_MINGW_ANSI_STDIO=1
     fi
 elif check_func_headers stdlib.h _get_doserrno; then
     libc_type=msvcrt