OpenWatcom is an open-source C/C++ compiler that can compile for a variety of systems, particularly legacy 16/32-bit Windows and DOS systems. This can be of interest for retro gamers and those using old devices that have DOS-based software, including industrial controllers and two-way radio programming.
CMake supports OpenWatcom, and is conveniently used with a toolchain file. GitHub Actions has easy scripting for OpenWatcom to test building of DOS programs from Linux.
The easiest way to show this is by example: see the Ascii Patrol game for how to build with OpenWatcom for DOS from Windows/Linux and GitHub Actions build CI.