enforce -Werror (if gcc) in hawq?

[Paul Guo]

-Werror
Make all warnings into errors.
I've seen many cases (not just hawq) before that ignoring gcc warning leads
to bugs. I'm wondering we should add the option for the gcc case. Given
there may be a lot of warnings when building the common postgres code in
hawq, we could at least enforce it in our own code at first
(src/backend/cdb, src/backend/resourcemanager, src/test/feature, other
directories?)? Any suggestion?