mozart: refactoring

- Append emacs to the oz wrapper's command search path rather than the
  rpath.  Previously, emacs would end up in the closure but the oz
  shell script would not be helped by it. Now a user without emacs in
  their PATH can still get the complete Oz experience (which depends
  crucially on emacs).  To build a variant without emacs, do
  mozart.override { emacs = null; }
- Patch full path to oz executable into the oz desktop item to make the
  output less reliant on the runtime PATH
- Compress .elc files to save a little bit of space
- Make it easier to extend platform support
- Inline builder.sh
- Be more specific about patching.  oz and ozc are capable of inferring
  OZHOME themselves; thus we generate wrappers only for the binary
  executable components.

Note that gmp and boost would be removed by patchelf --shrink-path; I've
no idea whether they are used somehow, so we leave them in and forego
rpath shrinking for now.