add Makefile for local builds

[matthijskooijman]

This is just a convenience for building an osf file. I could not find any instructions or script for it, so I created a Makefile for it. Should be pretty self-explanatory for anyone who used make before. I originally created this just for me, but it seemed worthwhile to share. Feel free to merge it, adapt it or ignore it. It might be good to add a note to the README, but I only know English :-)