My Linux work on Serious Sam is now merged. You can grab it here.
This has a bit to go yet. I had to abandon the file i/o code because it had changed so much (for the better, although no less incompatible), plus there's bound to be some mismerges, incompatibilities with newer GCCs and Clangs, and a dozen other things. The Makefile is attrocious too. But it's a big big push towards this running on Linux and Mac OS X again.
I'll accept pull requests, if anyone has any, with the eventual goal of pulling all this into the official Croteam git repository. When I get a little time, I'll start pushing it through modern compilers and get this going, but at least this is out there now. Feels good. :)
Thanks to Alen Ladavac for helping get this out there!