Use system libpcap when configured with --with-system-pcap

Don't force the local libpcap build when the system provides one. When
--with-system-pcap is given to configure, don't try to locate a local libpcap
build. This help build systems like Buildroot that store build trees in the
same directory, but still prefer dynamically linking against system wide
libpcap.so to save space.

Signed-off-by: Baruch Siach <baruch@tkos.co.il>