IF YOU GOT HERE, YOU PROBABLY DON'T WANT THIS PROJECT, YOU PROBABLY WANT THIS ONE: https://github.com/cnlohr/tsopenxr/
Minimal OpenXR Demo. Very minimal demo. It's mostly based on https://github.com/hyperlogic/openxrstub/blob/main/src/main.cpp.
Tested working on Windows + SteamVR, but should work elsewhere, since it's based on rawdraw.
But, it is pure C, compiles with TCC, or GCC in Windows. And is very minimal (no reliance on SDL or anything like that).
NOTE: this may not be well maintained, I recommend trying to use https://github.com/cnlohr/tsopenxr