[Rivet] Rivet 2.1.0 pre-release build test

[David Grellscheid]

> PS. Please note that the new Rivet does not use an external yaml-cpp
> library any more. So you will probably need to remove the
> --with-yaml-cpp switch from your build script.

Is that removed from autoconf already? There's no need to provide for
external YAML, and the potential confusion is too great. I can take it
out if not done yet, and you agree.

  David