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.
PR: 270788
Reported by: Fredrik Eriksson