Add META.json for PGXN release.

This file allows plpgsql_check to be uploaded to and distributed on PGXN. See [the HOWTO](http://manager.pgxn.org/howto) for details; get an account [here](http://manager.pgxn.org/howto). Click "Request Account" in the left nav to get an account if you don't already have one.

I've also changed the `Makefile` to prefer PGXS without the use of `USE_PGXS`, as that's the recommended way to build extensions outside the Postgres core. I've added `NO_PGXS` in case you want to do builds from the contrib directory of the Postgres core.