build: avoid YACC typo inherited from Autoconf.

The typo shows up in the `configure --help' output.  Reported by
W.C.A. Wijngaards and Paul Eggert at
<http://lists.gnu.org/archive/html/bug-bison/2011-07/msg00000.html>.
* README-hacking (Release Procedure): Remind about updating
maintainer tools before a release.
* THANKS (W.C.A. Wijngaards): Add.
* configure.ac (AC_PREREQ): Set to 2.68, which fixes the YACC
typo.
(cherry picked from commit 845346b3bb526d0f18878f15d906f05205b5c618)