It's a bit ugly to depend _only_ on the OpenBSD make infrastructure; it's better when things are portable! Making mymenu-portable would be more effort than what it's worth it, so just make the main version portable (again). kristaps@' oconfigure (used also by mandoc-portable and mymenu) is a lightweight configure system that's really nice to use. see GitHub issue #1

let's use OpenBSD' bsd.xconf.mk and bsd.prog.mk

don't pretend this is C89, this is C99. We use stdint and other stuff. This also remove A LOT of warnings etc...