xpra ebuild should honour the server flag and avoid building all the x stuff if disabled

[Antoine Martin]

The flag is there, it's just not used at the moment.

I can't figure out the standard way to pass arguments to a setup.py script.
Ideally, we could do:

./setup.py --no-server

And avoid building all the X stuff (which pulls zillions of dependencies)

[Antoine Martin]

done in r3566