intentional missing prerequisites?

[Canham, Timothy K (316C)]

We have a modular make system where we would like to be allow users to add and 
remove build targets.

What I would like to be able to do is something like this:

big_target: target_1 target_2 target3


target_1:
        do something

target_2:
        do something else

and target_3 is not defined anywhere.

I would like to know if there is a way to make the definition of the 
prerequisite target optional, and not break the rule. I know I could do:

target_3:

and it would work, but that would require adding hundreds of them, so I'd like 
to avoid it.

Any ideas?

Timothy K. Canham
Jet Propulsion Laboratory
Pasadena, CA
address@hidden