xfce.xfce4-dev-tools: Allow skipping xdt-autogen

This is useful since Xfce likely starts porting stuff to meson
but can still need xdt-csource for ui files.

pkgs/desktops/xfce/core/xfce4-dev-tools/setup-hook.sh

···
     NOCONFIGURE=1 xdt-autogen
 }
 
-preConfigurePhases+=(xdtAutogenPhase)
+if [ -z "${dontUseXdtAutogenPhase-}" ]; then
+    preConfigurePhases+=(xdtAutogenPhase)
+fi