Remove second AC_PREREQ, require version 2.60

Remove a second AC_PREREQ and bump the required autoconf version to
2.60.

Version 2.59c was a testing release, published in April 2006. Version
2.60 was the stable release afterwards, released in June 2006.

Signed-off-by: Daniel Martin <consume.noise@gmail.com>
Reviewed-by: Alan Coopersmith <alan.coopersmith@oracle.com>
Reviewed-by: Uli Schlachter <psychon@znc.in>
Tested-By: Ran Benita <ran234@gmail.com>