Add code to enforce the paralleism count (-j N) for the entire tree

Add code to enforce the paralleism count (-j N) for the entire tree
of submakes spawned during processing.

We create a fifo and stuff one character into it for each job we are
allowed to run.  The name of the fifo is passed to child processes
in the MAKE_JOBS_FIFO environment variable.

A make which finds this variable on startup will open the fifo and
only spawn jobs when it managed to read a token from the fifo.
When the job completes a token is writen back to the fifo.

Slave make processes get one token for free: the one their parent
make got in order to run them.  This makes the make processes
themselves invisible in the process counts.

The net effect is that "make -j 12 -s buildworld" will start at
most 12 jobs at the same time, instead of as previously up to
65 jobs would get started.

Taken-from: FreeBSD
Author: phk