cmake HAVE_PCAP_DEBUG examples

CTSRD-CHERI/cheribsd contrib/tcpdump/CMakeLists.txt :940 

check_variable_exists(pcap_debug HAVE_PCAP_DEBUG)

CTSRD-CHERI/cheribsd contrib/tcpdump/CMakeLists.txt :943 

endif(NOT HAVE_PCAP_DEBUG)