This patch adds the option -J / --auto-jobs, which uses the sysconf(_SC_NPROCESSORS_ONLN) function to decide the number of job slots to use.

This is useful in distributed build farms, where you have a large set of different machines where you want to take advantage of all machines' multiprocessing cababilities without getting the memory penalty of unlimited -j.

With -J we can set the flag in the Makefile and know each machine will do the best it can. Without -J we have to set a custom -j number in each and every box.