Log Message:
Update games/exchess to v7.31beta, this package's first update in 14 years.

The primary motivator for the upgrade is to improve the book generation
time, which is now several times faster (builds were failing with CPU ulimit
of 1 hour on 3GHz Xeon E5's with the previous version).  This new version
even supports multiple cores, though that isn't enabled at this time.

There appear to have been many other updates, but there is no ChangeLog
and there aren't even any intermediate versions for download (hence using
the current beta version).  Upstream unfortunately makes it as difficult
as possible to see the history of this package.