apple: build.sh: rework MAKEFLAGS jobs count

Forward the global MAKEFLAGS but adds the job count as is:
 - pass the number of core by default to keep the current behaviour of
   the script.
 - if the number of core is forwarded to MAKEFLAGS, use them instead of
   the previous.
 - if the number of core is explicitly given to the buildscript through
   the -j option, use them instead of the previous.
17 jobs for !32 with pending-patches-2 in 40 minutes and 20 seconds (queued for 3 seconds)
merge request