matchbox: depend on libXext at configure time

package/matchbox/matchbox.mk

@@ -211,7 +211,7 @@ else
 endif
 
 
-$(MATCHBOX_LIB_DIR)/.configured: $(MATCHBOX_LIB_DIR)/.unpacked
+$(MATCHBOX_LIB_DIR)/.configured: $(MATCHBOX_LIB_DIR)/.unpacked xlib_libXext-install-staging
 	(cd $(MATCHBOX_LIB_DIR); rm -f config.cache; \
 	$(TARGET_CONFIGURE_OPTS) \
 	CFLAGS="$(TARGET_CFLAGS) -I$(STAGING_DIR)/usr/include" PKG_CONFIG_PATH="$(STAGING_DIR)/usr/lib/pkgconfig" \