Document required autotools versions

CentOS 6 is roughly the oldest thing we care about developers still
using, and it has autoconf 2.63 / automake 1.11.  These are both
older than openssl 1.0.0, so anybody who can't upgrade past those
probably can't upgrade to a modern openssl either.  And since only
people building from git or editing configure.ac/Makefile.am need to
use autotools, I'm not totally enthused about keeping support for
old ones anyway.

Closes ticket 17732.