PSA: Autoconf has risen from the dead

While chasing something else, I was surprised to learn that the
Autoconf project has started to make releases again.  There are
2.70 (2020-12-08) and 2.71 (2021-01-28) versions available at
https://ftp.gnu.org/gnu/autoconf/

Right now, I'm not sure we care; there seems to be more
enthusiasm for switching to meson.  But if that idea falls
through, we should update to a newer autoconf release.

            regards, tom lane