handbrake: fix Darwin build by turning off GUI support

This involved several pieces:

- Always disable GTK GUI support under Darwin. The gtk3 package depends
  transitively on dbus, which depends transitively on systemd, which is
  not currently supported on Darwin. (I gather that it may be possible
  to work around this in the future.)
- Also disable the native GUI support under Darwin (using the
  --disable-xcode flag). Building this GUI would require using the Xcode
  build system, which I was not able to figure out how to do; for now,
  all builds on Darwin are command-line-only.
- Add the lzma package as a dependency on all platforms.
- Add dependencies on the AudioToolbox, Foundation, libobjc, and
  VideoToolbox packages on Darwin.