Prevent accidental deletion of the whole filesystem if make isn't executed properly.
I do realize that my following request might seem odd, but I just had a user doing the following:
bash Makefile html
This resulted in executing "rm -rf $(BUILDDIR)/*" after a few errors first and happily removed quite a bit (Since BUILDDIR isn't set when run through bash).
While this is obviously the users fault it would be nice if we could require the variable to be set before removing everything.