Build now emits a warning on unclean git directory

How to reproduce:

1. Make changes to a .scad file
2. Run ./build.py

Gives a warning:

Warning! Git repository is not clean:
M openscad/main_body.scad

Which looks like is is piped in from Git. I don't think this is a helpful warning to display as there isn't really anything to worry about and could get really annoying if you are working on something. Is it ok to hide this warning unless we are running with --force-clean @julianstirling ?