Create script to generate Unix source distribution package

[c-lipka]

We should have a script that we can run on the "raw" source package to create a Unix-specific distribution package; such a package should have the following features:

• does not require users to run unix/prebuild,sh (effectively has the prebuild step already performed on it)

• has anything stripped that is not required for installing on Unix after the prebuild step, most notably:

  • distribution/platform-specific/windows/
  • libraries/
  • source-doc/
  • tests/
  • tools/
  • windows/
  • unix/prebuild.sh
  • any "originals" of files generated by the prebuild step

  (There are a few more Windows-specific files tucked away rather deep in the tree, but given their limited number and size they might not be worth the effort to trim away.)

My personal suggestion would be to integrate this into the unix/prebuild.sh script, to be triggered via an option, and make it generate a compressed tarball.

[c-lipka]

See [.github/workflows/release_build.yml]() for a possible implementation.