devel/libgusb: Unbreak build
Register a dependency on py-setuptools. Somewhere in the dependency
chain a hard dep on it probably got dropped.
While here, use OPTIONS helpers, and adjust some build dependencies.
gtk-doc is only needed when DOCS is enabled, and vala is only needed
when VAPI is enabled.
Based on a patch from Matthew Wener.
Reported by: Fredrik Eriksson