On FreeBSD, one must actually link against libpcap
(or give the path to a shared object at link
time). That is, it is not a header-only library
on that platform.
Account for this by adding a new boolean variable
in `platform-quirks.cmake`: `NEED_PCAP_LIBRARY`,
which defaults to false. Condition pcap library
discovery and link argument modifications on that
variable in `FindPCAP.cmake` and `dep-link.cmake`.
0e985346b1