[plt-scheme] distribution build process?

I'd like to learn about the build process of the various distributions
of MzScheme and PLT available on the following pages

  http://download.plt-scheme.org/
  http://download.plt-scheme.org/mzscheme/

I imagine these packages are built from the following source code

  http://svn.plt-scheme.org/plt/tags/v4.2.1/

I don't see a top-level Makefile in that tag directory that combines
various parts of the four directories listed there into a tarball.

Any information about the process would be appreciated.

Thanks,
Peter