Multi threaded prescanning (with Ninja or Make)

[mls-m5]

For the moment presecanning is done on a single core (in contrast to actutal compilation). This could cause slow downs for bigger projects.

Considerations

I have a thought that you should be able to run matmake prescanning as a separate process. No real reason really, it would just be nice to break stuff out a bit. Consider if prescanning could be run with some special command line arguments, or if prescanning can be run as a separate program

[mls-m5]

I have changed my mind. The easiest way is probably to generate a separate build.ninja (or Makefile) file with rules for preprocessing, and run that before the main build.ninja is executed.