Well this is interesting!
>> make -j$(($(getconf _NPROCESSORS_ONLN)+1))
>
> Not really a good idea as it may break on users who have many cores
make ${MAKEFLAGS:-${NUMJOBS:-j2}}
or, for shits and giggles,
make ${MAKEFLAGS:-${NUMJOBS:-j$(($(getconf _NPROCESSORS_ONLN)+1))}}
(Thanks Johannes, I didn't know about getconf!)
-D.