deterministic builds using gitian

[leif]

It would be good to have the official packages of Tahoe and all of its dependencies built using Gitian.

From ​http://gitian.org/:

Gitian uses a deterministic build process to allow multiple builders to create identical binaries. This allows multiple parties to sign the resulting binaries, guaranteeing that the binaries and tool chain were not tampered with and that the same source was used. It remove the build and distribution process as a single point of failure.