use-system-binary option

Good day,

I've been in a need to serve my buildout-based app with a system uwsgi binary (installed from the OS repositories)

I've added an option to buildout.recipe.uwsgi to skip downloading and building uwsgi but still to generate the xml config which can then be used with the system uwsgi.

This is a simple one-line change and does not affect the behaviour of existing buildouts.