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.