Adding in -j option to the build script.

Before this option was hardcoded as always on and just looked at the
machine to figure out how many processors are available. I have now set
this as a configurable option on the command line of the build script.
If nothing is supplied, the number will be gotten from the machine. If
-j <int> is supplied, that number will be used. 0 is the default value
and triggers the searching of the machine.
Adding in case where 0 processors found.

I'm not really sure this will ever be a problem but I'm adding in the case
where 0 processors are found on the machine. If this happens, I default to
1 processors for the build. I don't think this will happen but on new archs
I suppose it could, especially if the commands I use to get the processers
don't work.
Moving numProc check to its own function.