Semi-automatic update. These checks were done:
- built on NixOS
- Warning: no binary found that responded to help or version flags. (This warning appears even if the package isn't expected to have binaries.)
- found 3.1.7 with grep in /nix/store/8r3k0r8vf0wfpj14443pm1k0jddlnii2-shairport-sync-3.1.7
- found 3.1.7 in filename of file in /nix/store/8r3k0r8vf0wfpj14443pm1k0jddlnii2-shairport-sync-3.1.7
- Enable additional backends: pipe, stdout
- Support high-quality resampling with soxr
- Support configuration files, but don't install one
- Cut down on boilerplate code with fetchFromGitHub/autoreconfHook
