Makefile

This was originally https://github.com/kfogel/OneTime/pull/12 -- a PR contributed by DomT4 (thanks, Dominyk!). It's closed now.

I'm reflecting all the GitHub OneTime issues over here at code.librehq.com, preserving ticket numbers. For closed tickets like this one, I'm not bothering to copy over any of the details. (There's a way to import them all properly into GitLab, but there aren't enough issues to make it worthwhile to learn how to do that. Instead, I'm just copying them over by hand.)