synergy: Use bundled crypto++ on non-Linux archs.

Seems that crypto++ in nixpkgs doesn't build on Darwin, so let's use bundled
crypto++ until the version in nixpkgs works well.

This refers to the following build:

http://hydra.nixos.org/build/5404516

Hopefully, this will fix it on Mac OS X, because I don't have a Darwin machine
for testing.

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