The perl code in src/test/regress was using a mix of either not
printing the version, printing it wrong (due to us not using CVS
anymore) or using a hardcoded string. Implement a new module for
common test code called GPTest.pm which abstracts this (for now
it's the only thing it does but this might/will change, hence the
name). The module is created by autoconf to make it pull in the
GP_VERSION from there.

While there, simplify the version output in gpdiff which included
the version of the system diff command - somewhat uninteresting
information as it's not something that changes very often and just
cluttered up the output.

This removes the MakeMaker support but since we have no intention
of packaging these programs into a CPAN module it seems pointless
to carry that format around.