Fix CMake checks of PCap functions, along with AirPCap

Change-Id: If55498167d5e357258841f2194962749c0dfc81e
Reviewed-on: https://code.wireshark.org/review/3787
Reviewed-by: Graham Bloice <graham.bloice@trihedral.com>
Petri-Dish: Graham Bloice <graham.bloice@trihedral.com>
Reviewed-by: Roland Knall <rknall@gmail.com>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Jeff Morriss <jeff.morriss.ws@gmail.com>