contrib: recursive dependency tracking

Let's say we're building gnutls (since the system version is too new).
gnutls depends on nettle.
Let's say we're using the system nettle (naturally in FOUND_PKGS).
nettle depends on gmp.

With the old (non-recursive) dependency tracking, we would end up building
gmp just for nettle, even though we aren't even building nettle!

Change-Id: I24e6f59416839ea202e0aceafe101432fe9c2646
Reviewed-by: Guillaume Roguez <guillaume.roguez@savoirfairelinux.com>