[plt-scheme] don't -c with setup-plt

Scott Owens got tired of having to re-compile *every* .zo file after
updating plt/collects from CVS. After all, a change in one module
affects only a subset of all modules, and the module dependencies are
fairly obvious from `require' declarations.

Thanks to Scott's work, you should not use "-c" with Setup PLT anymore.
In fact, "-c" now implies "-n" (which means "don't compile any .zo
files").

Instead of relying on you to supply "-c" when a module changes, Setup
PLT keeps a timestamp-based .dep file next to any .zo file that it
creates. The .dep files track which modules depend on which other
modules. As a result, Setup PLT can re-compile a much smaller set of
.zos when any one module changes.

The don't-use-"-c" rule takes effect immediately when you update your
plt/collects directory from CVS.

DrScheme will eventually get some sort of compilation manager, but we
need to iron out issues with debugging annotations (used to show the
control state for uncaught exceptions).

Matthew