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>

configure

@@ -5127,6 +5127,9 @@ if [ "$CFG_XCB" != "no" ]; then
         if [ "$CFG_XCB_XLIB" != "no" ]; then
             if compileTest qpa/xcb-xlib "xcb-xlib" $QMAKE_CFLAGS_XCB $QMAKE_LIBS_XCB; then
                 QT_CONFIG="$QT_CONFIG xcb-xlib"
+                CFG_XCB_XLIB=yes
+            else
+                CFG_XCB_XLIB=no
             fi
         fi