[compiz] adding XCB dependency

[David Reveman]

I intend to make compiz depend on XCB and Xlib/XCB sometime soon so we
can start using XCB where it's beneficial [1]. I'm mostly interested in
using it for asynchronous reading of window properties but we should
eventually try to use it for as much as possible. Removing Xlib
completely might not make sense as we need to support multiple
implementations of GLX but getting to a stage where it's possible to
disable Xlib at build time for those who are interested in that would be
neat.

If you've got any issues with XCB and Xlib/XCB being added as
dependencies please speak up asap. Thanks.

[1] http://xcb.freedesktop.org

- David