libsigc++ patch

I was really pleased to get a big libsigc++ patch a few days ago from Régis Duchesne at vmware, to fix some crashes that affect a libsigc++ technique they are using. It required significant time, and a deep understanding of both libsigc++ and C++ templates in general. In mature projects, maintainers are often left to fix the difficult/not-fun stuff themselves, so it's uplifting when somebody takes some of the weight off. Thanks Régis.