Remove NotImplementedError for MinGW-w64

See #3405 for discussion about this issue.

Some (me, @mwtoews) have successfully built Fortran extensions using the MinGW-w64 compiler. The block affected by this PR was written by @cournape in dfefe95, and perhaps the line "# XXX: fix this mess, does not work for mingw" is worth removing at this point too.