Make sure we can link with libpcap.

Make sure we can link with libpcap.

After finding libpcap and getting a list of other flags needed when
linking, we check for "pcap_loop()", to make sure we found a working
libpcap and have all the right other libraries with which dnl to link.
(Otherwise, for example, the check for pcap_list_datalinks() will fail,
not because the routines are missing from the library, but because we
aren't linking properly with libpcap, and that will cause confusing
errors at build time.)