bar - Remove unnecessary dependencies and centralize version string.

Peti recently accepted a pull request for LemonBoy's Bar Ain't recursive from meisternu. I happened to be working on a derivation for that as well and might have some efficiencies.

@meisternu I see you pushed this derivation along with bspwm - I use herbstluftwm w/o xinerama. The changes build, link, and install cleanly but I wanted to make sure I didn't miss anything you saw.