We are a GNU project

[Peter Simons]

Hi guys, 

in case you haven't noticed already anyway, check out what Savannah says on
our project home page <https://savannah.gnu.org/projects/autoconf-archive>: It
says "This project is part of the GNU Project." loud and clear.

That feels pretty nice. And, of course, that transition comes with the added
benefit of having an excuse for changing every single one of our URLs again.
It's probably the 6th time by now that this happens. I'm sure our users love
it. :-)

Take care,
Peter