Require autoconf 2.60, as

version 2.59 seems to be incompatible with our updated macros.
If somebody insists on 2.59 (which is almost 10 years old) I'll
have to hunt down the reason for the recent (Dec 2012?) breakage.

Note that I did not check with 2.60 (I have 2.65), so that one
may still be problematic.