Ensures that config.h is always ordered correctly (first) in the
includes. Also means that new source files get this for free without
having to remember to add it.

We opt for -imacros over -include as its more portable, and the
added constraint by -imacros doesn't bother us for config.h.

This also touches the HACKING file to remove the explicit mention of
config.h as part of the includes.

Signed-off-by: Dave Reisner <dreisner@archlinux.org>
Signed-off-by: Dan McGee <dan@archlinux.org>