synergy: Add support for XrandR.

Nowadays, multiple monitor setups are quite common, so I suppose we'd want
support for that. Especially because users might get confused if synergy is
unable to pick the right screen resolution and thus cause edges to be cut off
from the available pointing area.

The postPatch hook is to force cmake into thinking that we have XRRNotifyEvent,
which we _do_ have with the xrandr version shipped in nixpkgs. Automatic
detection from CMakeLists.txt fails here because it tries to search for the
symbol within the libX11 store path.

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