graphics/netpbm upgrade from _2 -> _3

[Matthias Andree]

Uh, I just figured that is partially unintelligible.

So here goes:

You currently get in ports/graphics/netpbm:

- a version 10.35.80_3 that can be built even with the old netpbm
version installed (no matter if 10.35.80_2 or 10.2?.*)

- this version still builds with only one job at a time, marked
MAKE_JOBS_UNSAFE=yes.

This resolves the immediate upgrade pains.


What has not been committed yet, from the same PR:

- a change that enables MAKE_JOBS_SAFE= and thus parallel builds on
multi-core computers.  It works for Dirk and me, so I expect we'll see
it as soon as Dirk gets around to it.

Best regards
Matthias