libvdpau: add an option 'x11' (default enabled) in available.mk, so non-X11
users can avoid all of Xorg being pulled in for a dependency with ease,
without needing to know about vdpau, for example in the case of ffmpeg3
which shouldn't require X11.

Feel free to propose alternatives.

Clean some commented out bits.

PR pkg/52021

(maya)