If you need this, you’re probably doing it wrong. But if you really must check to see if a variable has been assigned yet, there’s nothing really stopping you. Here’s one way of implementing it.

In GNU’s Make manual, there’s a description of how to use one Makefile to call targets of another Makefile. You have to use $(MAKE) in order for it to work properly. I’ve found one little snag that’s not mentioned. If my target name is the same as the directory name where the other Makefile is, it will never work. It always says it’s up to date! If I change the target name, it works fine.