The issue is here:
https://github.com/eidheim/tiny-process-library/blob/master/CMakeLists.txt#L12

It errors out trying to build process_unix.cpp

The CMake variable MSYS must presumably only be set under the MSYS makefile generator, and not under the ninja generator when used in the MSYS environment. (I don't recall seeing it before, so I don't know how standard/common/reliable of a variable it is to use)

If you assume that the user must use MSYS2 and cannot use MinGW on its own, then you could generalize the check to if(WIN32) - this is probably the simplest, best solution at least in the short term.