Aften is an audio encoder which generates compressed audio streams based on
ATSC A/52 specification. This type of audio is also known as AC-3 or Dolby®
Digital and is one of the audio codecs used in DVD-Video content.

Homepage: http://aften.sourceforge.net/

Motivation for this change

libjack2 in nixpkgs is broken on darwin. The configure phase fails with the following error:

Checking for library aften               : not found

So this PR adds the aften library to nixpkgs. Unfortunately, this is not enough to completely fix libjack2 (the configure phase goes through, but there are build errors related to CoreFoundation).

Things done

• Tested using sandboxing
  (nix.useSandbox on NixOS,
  or option build-use-sandbox in nix.conf
  on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • Linux
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Fits CONTRIBUTING.md.