Please review: proposed dependency handling change

[Joshua Root]

Ticket: <http://trac.macosforge.org/projects/macports/ticket/15161>

The full enhancement requested by the ticket will (AFAICT) be rather 
difficult to implement at present. I have however attached a patch which 
improves things a bit. It makes 3 things better:

1. You can use depends_build for software that needs to be used in the 
extract or patch phases. This helps e.g. python23 which needs gnutar for 
extract.
2. You can use depends_build for software that is needed for the 
configure phase but not at runtime (e.g. pkgconfig). Currently 
depends_build is not checked until after the configure phase.
3. Dependencies weren't being checked when running some packaging 
targets. Now they are.

- Josh