while here also add a license and mention the copyright on Makefile. It's now very different from the mandoc-portable one, but that was the base for this one (including how to `make dist').

see GitHub issue #1.

these are still overrideable from the args passed to the script.

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