Modest proposal: run check_keywords.pl on every build

I've had it with mopping up after oversights like this one:
http://archives.postgresql.org/pgsql-hackers/2012-09/msg01057.php

We made a script (src/tools/check_keywords.pl) to check for this type of
error years ago, and even added it to the top-level "maintainer-check"
target awhile back, but nonetheless people continue to screw it up on
a regular basis (twice already this year alone). I think we should move
the script into src/backend/parser and run it as part of the make gram.c
target. (Doing it that way will not cause perl to become required for
building from tarballs, since gram.c is supplied prebuilt in tarballs.
And we already require perl for builds from raw git pulls.)

Any objections?

regards, tom lane