specifying dependencies on projects without cps

One of the pain points of pkg-config is that specifying that a pkg-config dependency that has a non-pkg-config dependency is really awful. You basically just say "here's some -I -l and -L options, hopefully everything works out." It would be nice if CPS had a way to specify that it depended on non-cps dependencies. I see a couple right off the bat
- pkg-conifg
- cmake-config

Neither of these are going to go away just because CPS is suddenly released, at least not immediately.

- Apple frameworks
These are *never* going to go away, and we have no power over them. being able to add them as a dependency would be very useful.