build: improve pcap dependency handling

build: improve pcap dependency handling

pcap has historically shipped a custom pcap-config binary tool which
does the job of pkg-config. It was never compatible with cross
compilation.
Meson uses it when using dependency(), which then means cross
compilation fails.
Set pcap-config to empty in the meson cross compilation files so
that Meson will not use it, and add a fallback in case
dependency() fails.
libpcap 1.9.0 finally ships a pkg-config file so everything will
work out of the box in the future.

Signed-off-by: Luca Boccassi <luca.boccassi@microsoft.com>