synergy: Only pass cryptopp on Linux platforms.

The second failure, and the last one I'm going to try today:

http://hydra.nixos.org/build/5404634

On the bright side there is at least the fact that version 1.4.10 has failed on
Darwin already, so I guess we don't have a lot of Mac users using Synergy.

Latest (failed) build of 1.4.10:

http://hydra.nixos.org/build/5359408

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