[Inkscape-devel] cmake fails: unsupported Mingw

Bad result from cmake .. command:

Unable to determine MinGW processor architecture. Are you using an unsupported MinGW version?

I put MinGW exactly where it's needed - in C:\ and without whitespace characters. How do I fix this? There's nothing on Google that will explain it. I'm using the normal MinGW, not a legacy version or beta.