Missing libcap error is confusing

[dionyziz]

Making ncdns with a missing pcap dependency produces peculiar errors that are difficult to debug. An error indicating that "pcap is required, please run apt-get such and such" would be much more user-friendly.

[hlandau]

You're confusing libpcap with libcap. libpcap concerns packet capture. libcap concerns Linux capability sets. Anyway, maybe...