Real dist release tarballs

[apjanke]

We're currently relying on GitHub Releases's "download this tag" functionality for distribution tarbals. We should probably be creating real dist tarballs with a make dist target. To be a good FLOSS ecosystem citizen and avoid a hard dependency on GH publishing infrastructure. And for signing them, when we get around to that.

References

• Ubuntu Upstream Guide
  • its "tarball" section