Add support for ftp mirrors

[akotlar]

Currently it appears that Alien::gmake relies on ftp.gnu.org, which happens to be down right now. Having a mirror fallback would be awesome.

[plicease]

according to https://www.gnu.org/prep/ftp.html there exists https://ftpmirror.gnu.org/ which takes you to a nearby mirror. That might be the best thing to do in this case.