fix removal of {cpu,machine} links on mingw/msys

fix removal of {cpu,machine} links on mingw/msys

At least on Windows 2003 using "ln -s dir1 dir2" in msys shell succeeds,
but results in what looks like hard link. Subsequent "rm -f dir2" (e.g.
during second config.status invocation) fails. Check that we also can
remove link to directory.

Make it more clear in message that we are checking "ln -s".