I'm not absolutely sure this works.  cc and the tools that use it
(ProjectBuilder) make Mach-style binaries, not the old CFM-style ones.  I
think that external plug-ins in Squeak require CFM-style shared libraries.

I've built Carbon plug-ins in CodeWarrior without any problem, but I'm not
sure how to do it with cc/ProjectBuilder.

Carbon and OS X support a Plug-In API that I think could be used in
Squeak.  I also think it gets around this problem because building a
Squeak plugin would be like building a framework using ProjectBuilder.

For now, if you can, build Carbon plugins using MPW or CodeWarrior.

-Eric