Or if you don't like taeric's suggestion you can just touch a .ao file after the line that creates the .a and .o files and have your further rule(s) depend on that .ao file. Have .ao depend on your source. If you still want to be able to type stuff like 'make foo.a' instead of 'make foo.ao' and have it work, then you can make a rule where .a depends on .ao and all the rule does is touch the .a file. Create the same rule for the .o too.
