Set XR_OS_LINUX on Mingw?

When building OpenXR-SDK-Source on mingw, XR_OS_WINDOWS is defined instead of XR_OS_LINUX. This prevents the loader to use the XDG_DATA_DIRS envvar to find the API layer manifests.
I wonder if setting XR_OS_LINUX would have any “bad” side effects…

That’s not how you find the runtime or layers on Windows, however. MinGW builds windows binaries that should have windows behavior, which in this case means some registry paths are checked.