OpenMP on Darwin

[Jason Bacon <jtocino%gmx.com@localhost>]

The existence of Apple silicon has created more motivation to get
scientific packages working on Darwin by significantly increasing
computing power / cost compared with Intel Macs.

One of the problems is that Apple's clang lacks OpenMP support.  I see
parallel/openmp has a buildlink3.mk, but including it does not appear to
be sufficient to make packages work on Darwin.

Has anyone created any packages that successfully use OpenMP on Darwin?
I'm not seeing anything in the guide or anything obvious poking around
the pkgsrc tree.  If not, I'll start working on a reliable approach.
There are hacks posted on the web about manually installing OpenMP
libraries and using "-Xpreprocessor -fopenmp" and "-lomp".  I think
pkgsrc should be able to do something like this automatically.