configure: Properly report xcb-xlib configuration

d34cae51 introduced a new configuration parameter, xcb-xlib, but did not
remember to set the internal variable after the config test was run,
resulting in a potentially incorrect reporting of the option.

Change-Id: I979589d9aad2eb5e0fac069dfcd5b329a51ae059
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>
xcb qpa: initialize EGL for non-XLib builds

Move the EGL initialization code outside of the XLib ifdefs, so it
can be enabled for non-XLib builds as well

Change-Id: Ie025551e4e99bb0b365f025356bd9725f4283b82
Reviewed-by: Laszlo Agocs <laszlo.agocs@digia.com>