build: cleanup libpcap dependent components

build: cleanup libpcap dependent components

The RTE_PORT_PCAP variable is used to signal libpcap availability,
though its name seems to refer to pcap support in the port library.
Prefer a generic name and add explicit link dependencies where needed.

Fixes: 7a944656b33f ("test/pcapng: test pcapng library")
Fixes: 2eccf6afbea9 ("bpf: add function to convert classic BPF to DPDK BPF")
Fixes: cbb44143be74 ("app/dumpcap: add new packet capture application")
Signed-off-by: David Marchand <david.marchand@redhat.com>
Acked-by: Stephen Hemminger <stephen@networkplumber.org>