pcap: revert build patches

pcap: revert build patches

This reverts commits
a0cdfcf9 (use pcap-config to guess compilation flags),
ef5b2363 (fix build with empty LIBPCAP_CFLAGS) and
60191b89 (fix build when pcap_sendpacket is unavailable).

These patches are creating more problems than solving the initial one
(which was a build error with too old pcap libraries).
Since old pcap libraries are not that common, just revert them.

Reported-by: Meir Tseitlin <mirots@gmail.com>
Reported-by: Mats Liljegren <mats.liljegren@enea.com>
Signed-off-by: David Marchand <david.marchand@6wind.com>
Acked-by: Thomas Monjalon <thomas.monjalon@6wind.com>