Fix building on slightly older -current and stable systems after
the switch to bmake. The rescue bits are built via crunchgen,
which didn't respect the MAKE environment variable until r237574
(i.e. ~11 months ago). This resulted in a failure due to bmake's
internal -J flag being passed around and not being understood by
the standard (i.e. host's) make.
Note that the failure is conditional upon having the jobServer
feature enabled within bmake.