build: properly handle ./configure --with-libpcap
build: properly handle ./configure --with-libpcap

Without this fix, ./configure --with-libpcap will cause --with-libpcap=yes
to be implicitly passed down, which cause yes/bin/pcap-config to be
searched for rather than /usr/bin/pcap-config.

Also output pcap: no when pcap is not found or disabled.

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