configure.ac: Report if built with XCB-XKB or not

Signed-off-by: Jonas Ådahl <jadahl@gmail.com>
diff --git a/configure.ac b/configure.ac
index 84f3a79..5d0d060 100644
--- a/configure.ac
+++ b/configure.ac
@@ -506,6 +506,7 @@
 	Cairo Renderer			${with_cairo}
 	EGL				${enable_egl}
 	libxkbcommon			${enable_xkbcommon}
+	xcb_xkb				${have_xcb_xkb}
 	XWayland			${enable_xwayland}
 	dbus				${enable_dbus}