configure: update req to 2.59

I ran the 2.59 version of autoupdate that updates obsoleted configure.ac
constructs to the 2.59 standard. With a little hands-on fiddling I
prevented it from ruining the quoting in AS_HELP_STRING() uses.

I subsequently also bumped the required autoconf version to 2.59
(released in December 2003) as I don't have an older autoconf version
around to test with and I can't be bothered to install one either...

Inspired by: Björn Stenberg
Related blog post: http://cazfi.livejournal.com/195108.html