There is a big sale on various id Software games on GOG now, so I bought Return to Castle Wolfenstein there at last DRM-free, to replace my old CD version.

There are various projects which made originally released source code by id Software usable with up to date libraries like SDL2. The most promising and actively developed one is https://github.com/iortcw/iortcw

I got the source and tried building it on Debian testing (simply run make). There are two subprojects - single and multiplayer ones. I built both. You need various development packages obviously (like C compiler), and then SDL2 and the rest. On Debian testing for example make sure to install libsdl2-dev.

When building the source, it failed with not finding GL during linking. Apparently it's caused by Nvidia driver installation in some way. I was able to work around it by doing:

sudo ln -s /usr/lib/x86_64-linux-gnu/libGL.so.1 /usr/lib/libGL.so

If you are in the same situation, you can remove that symlink later, after building. You also need original game files, which you can get on GOG, or may be you already have them from other sources. Here are files that were produced for single and multiplayer versions:

SP/build/release-linux-x86_64/iowolfsp.x86_64
SP/build/release-linux-x86_64/renderer_sp_opengl1_x86_64.so
SP/build/release-linux-x86_64/renderer_sp_rend2_x86_64.so
SP/build/release-linux-x86_64/main/cgame.sp.x86_64.so
SP/build/release-linux-x86_64/main/qagame.sp.x86_64.so
SP/build/release-linux-x86_64/main/ui.sp.x86_64.so

MP/build/release-linux-x86_64/iowolfmp.x86_64
MP/build/release-linux-x86_64/iowolfded.x86_64
MP/build/release-linux-x86_64/renderer_mp_opengl1_x86_64.so
MP/build/release-linux-x86_64/renderer_mp_rend2_x86_64.so
MP/build/release-linux-x86_64/main/cgame.mp.x86_64.so
MP/build/release-linux-x86_64/main/qagame.mp.x86_64.so
MP/build/release-linux-x86_64/main/ui.mp.x86_64.so

Place those files from */build/release-linux-x86_64/ and */build/release-linux-x86_64/main into corresponding directories where you placed you game (if it's from GOG, you can unpack it with innoextract and copy stuff from "app" directory to desired destination, and you'll have to rename Main to main, since it's not lowercase there).

I also patched the game to use XDG base directory specification, in order not to pollute $HOME. You can find patched version here. Hopefully they'll accept that upstream.

In order to enable widescreen resolution, find config files. In the patched version it's normally
$HOME/.local/share/return-to-castle-wolfenstein/main/wolfconfig.cfg
$HOME/.local/share/return-to-castle-wolfenstein/main/wolfconfig_mp.cfg
for single player, and multiplayer accordingly (unless you have explicit XDG_DATA_HOME defined, then look there). Set these values:

seta r_mode "-2"

This will use your native desktop resolution. Or you can set it explicitly to any resolution you want like this (for example 1920 x 1200):

seta r_mode "-1"
seta r_customwidth "1920"
seta r_customheight "1200"

Those config files are created after you start the game for the first time. So modify them after exiting that first run.