cairo: xcb implies x11

Hi!

The cairo package has an xcb option. When it's enabled, it also
does everything the x11 option does.

This is a problem if you want to turn off x11 with '-x11' in the
options - if you don't turn off xcb as well, nothing happens.

I'm not quite sure why xcb even is a separate option. So here are
proposals:

1. Merge the xcb option into the x11 option (effectively removing it),

or

2. Make it an error to enable the xcb option while the x11 option is
   disabled.

Comments, suggestions?
 Thomas