Re-add MAKE_JOBS_UNSAFE=yes

It is not clear that the issue that caused this to be added has been fixed, so
add it back to be safe.

Reported by: jmarino (via ports-committers@), mat (via phab diff D7703)
Reviewed by: swills (mentor)
Approved by: swills (mentor)
Differential Revision: https://reviews.freebsd.org/D8086