[plt-scheme] mzc --auto-dir and --3m don't work together

Hello,

Using mzc --auto-dir --3m <some-extension.scm> results in a library 
installed in the typical collects directory:

[output to "./compiled/native/ppc-macosx/unsafe.dylib"]

whereas it should output to

[output to "./compiled/native/ppc-macosx/3m/unsafe.dylib"]

Thanks,
Will