Revert "Remove the "lib/%" rule."

This reverts commit 6fc43ddaf6.

I didn't think this through before pushing it.

Makefile

@@ -554,6 +554,10 @@ test: test-all
 .PHONY: test-clean
 test-clean: test-all-clean
 
+lib/%:
+	git submodule sync $?
+	git submodule update --init $?
+
 git-submodule:
 	git submodule sync --recursive
 	git submodule update --init --recursive