If you have time, can you check CMake version?
https://github.com/stalkerg/postgres_cmake
During Mingw installation you can choose a version, it can be more MSVC
like or more Unix, probably Postgres have a problem with one of mingw type.
So I unpacked the source tarball within the shell itself(using gunzip and
> tar), and I could fix many errors.
>
This is really stange, mingw on windows should't depend on any attribute.