[Simh] Makefile performance comparison

[Lars Hamren]

I was a little puzzled at first; there did not seem to be any
explanation to the speed differences.

After some inspection, I found that there is one significant
difference between Philipp's and my makefiles:

      - Philipp does not compile all files for every simulator. Instead
        files like sim_*.c, and others, are compiled only once, and the
        object files are reused.

      - I take a more cautious approach and recompile all the source
        files for each simulator, in the same way that the standard
        simh makefile does.

Philipp's approach is of course faster, but requires certainty
about which source files depend on simulator-specific defines,
and which do not.

On the subject of the number of parallel jobs to use, I have found that
using twice as many jobs as processors is a good approach. More jobs
will bring no benefit, and fewer may fail to saturate the CPU.

/Lars