If we don't have any support for packet capturing, don't bother

If we don't have any support for packet capturing, don't bother
supplying a non-stub version of "pcap_findalldevs()", as a list of
devices won't do any good for a capture program.

Don't bother checking for "ifaddrs.h" unless we have "getifaddrs()" -
and if we have "getifaddrs()" but don't have a usable "ifaddrs.h",
report an error, rather than leaving the error for the compile stage.