science/mbdyn: Unbreak build by properly implementing get_nprocs() for FreeBSD.

It looks like it was only built for one CPU before and didn't fail.
Some change in the underlying libraries triggered the build to become
multi-CPU and it began failing because get_nprocs() wasn't available
for FreeBSD and also was't properly linked to the surrounding code.
Reported by: fallout

(cherry picked from commit c4254019d8e8aa636db96bcea5702e0a39880048)