port: fix pcap support with meson

port: fix pcap support with meson

The meson build was missing the define to enable pcap port support if
libpcap (development) package was found on the build platform. Rather than
duplicating the checks for libpcap found in the pcap net PMD build file, we
can move the checks to the top-level config directory and reference the
RTE_PCAP_PORT setting elsewhere in the build.

Bugzilla ID: 351
Fixes: 5b9656b157d3 ("lib: build with meson")
Cc: stable@dpdk.org
Reported-by: Cristian Bidea <cristian.bidea@keysight.com>
Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>
Tested-by: Cristian Bidea <cristian.bidea@keysight.com>