synergy: Update to new upstream version 1.4.12.

This brings in support for encryption and thus requires the crypto++ library as
an additional dependency. Unfortunately the upstream integration isn't quite the
way we'd like it to be, so we need to add a small patch to ignore the bundled
version and use the package from nixpkgs.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>