Add new configure parameter for Xcb-Xlib

This adds a new configuration parameter, xcb-xlib, which allows
overriding the configure test for that existing configuration option.

The use of xcb-xlib in the xcb platform plugin becomes the preferred path
for non-OpenGL ES 2 builds, while the EGL codepath is used otherwise.
This has the advantage that EGL can be used with Desktop OpenGL if Qt is
configured with -no-xcb-xlib.

Change-Id: I5018e31fe0399b94f020c671eff9414d00431c44
Reviewed-by: Jørgen Lind <jorgen.lind@digia.com>
Move the glxfbconfig configtest to qpa and rename it to glx

We require glx version 1.3 which is where fbconfig is first defined.
Also make use of the configure test and report the glx status.
GLX support should always take precedence when compiling the
xcb backend

Change-Id: Ie46834210bf5cd2ac4006ff08379e0d3434ffa2b
Reviewed-by: Laszlo Agocs <laszlo.agocs@digia.com>