CMake: Add conditional builds.

In my version of XCode ranlib complains bitterly and returns an error if
a module has no symbols. Make the AirPcap, RTP player, and remote
capture code conditional. We could also try passing
"-no_warning_for_no_symbols" to ranlib but we probably shouldn't try to
comple the code in the first place.

Change-Id: Ide4f3d649eac354d54f0dde27dbc8e9cb270c65f
Reviewed-on: https://code.wireshark.org/review/4220
Reviewed-by: Gerald Combs <gerald@wireshark.org>
Petri-Dish: Gerald Combs <gerald@wireshark.org>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Evan Huus <eapache@gmail.com>