cmake HAVE_PF_RING examples

MRPT/mrpt cmakemodules/FindPCAP.cmake :49 

check_function_exists(pcap_get_pfring_id HAVE_PF_RING)

FreeRTOS/FreeRTOS-Plus-TCP cmake_modules/FindPCAP.cmake :67 

check_function_exists(pcap_get_pfring_id HAVE_PF_RING)