Pull code down from source code control, Google has some excellent copy/paste instructions for this. See http://code.google.com/p/ohboy/source/checkout and http://code.google.com/p/gnuboy/source/checkout quick instructions below:
hg clone https://ohboy.googlecode.com/hg/ ohboy cd ohboy svn checkout https://gnuboy.googlecode.com/svn/trunk/ gnuboy
Unlike gnuboy which uses the Autoconf (configure/make) system, Oh Boy uses regular make which makes platform porting fairly easy, even to platforms where autoconf is not an option. Determine the make file for your platform an issue:
make -B -f Makefile.PICK_ONE
Also see ToDo list.
Merge the SDL port, i.e. remove the duplicated functions (pcm_submit, vid_init, etc.).
Merge functionality into gnuboy.
By replacing Freetype2 using SFont and either using BMP files (SLD base BMP support built in) or adding built in support for TGA or png via code from http://www.nothings.org/ (thanks to http://www.philhassey.com/blog/2010/01/05/seahorse-adventures-loading-tgas-and-more/ for pointing me in that direction). There is always SDL_Image too.