Largely inspired from https://build.opensuse.org/package/view_file/science/glpk/glpk.spec but a fix was needed to make it work on Windows; see https://lists.gnu.org/archive/html/bug-glpk/2013-06/msg00012.html

Could the documentation files go rather somewhere into the %{_mingw64_prefix}. I don't like the native windows packages populate anything beyond the sysroot

How can I do that ? By replacing all the %{_docdir} by %{_mingw64_prefix} ?