Encouraging ports to include use_parallel_build

[Ryan Schmidt]

Of our 5256 ports, 326 (6.2%) indicate they can build in parallel,  
and 33 (0.6%) indicate that they cannot. That leaves 4897 ports  
(93.1%) which do not indicate whether a parallel build is possible or  
not, and in this case, MacPorts will assume a parallel build is not  
possible.

I would like to encourage port authors to indicate in their portfiles  
whether parallel building is possible or not. I'm considering whether  
we should add a check for this to "port lint". Hypothetical output:


$ port lint giflib
--->  Verifying Portfile for giflib
Warning: Consider adding use_parallel_build
--->  0 errors and 1 warnings found.
$