[plt-scheme] mzc --make

I've added a "--make" (= "-k") flag to mzc. It compiles Scheme source
modules to .zo in the same way as Setup PLT: it recurs to compile
imported modules, it uses/creates .dep files, and "--auto-dir" is
implied.

For most purposes, use "mzc --make" instead of "mzc --zo".

(This seems like an obvious improvement to mzc, and I'm not sure why we
haven't done it before.)

Matthew