Load Xlib functions from Xlib instead of Xlib-xcb; fixes #4

Not all distributions link libX11-xcb.so against libX11.so, hence
loading ``XOpenDisplay`` and ``XCloseDisplay`` from libX11-xcb.so may
fail.

This patch loads all Xlib symbols from libX11.so and only
``XGetXCBConnection`` from libX11-xcb.so, and adds dependency
requirements to make sure Xlib provides and is compiled with XCB
transport.