provide a sensible fallback for compat_getprogname.c

include config.h in all compats

switch to kristaps@' oconfigure 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