winbuild.sh: ac_cv_func_working_vsnprintf
Set ac_cv_func_working_vsnprintf in winbuild.sh, so that the build will not use the fallback vsnprintf() implementation, as is the default for a cross-compilation.
Well, the vsnprintf() really is not suitable for us, so we have to use the fallback.
Set ac_cv_func_working_vsnprintf in winbuild.sh, so that the build will not use the fallback vsnprintf() implementation, as is the default for a cross-compilation.