Try the pcap_dump_ftell() check after pcap-config.

Try the pcap_dump_ftell() check after pcap-config.

Apparently, the test for pcap_dump_ftell() cannot succeed if the test
program isn't linked with libpcap, this depending on the output of
pcap-config. That's why all pcap_* function checks come after the
pcap-config check.

This explains why in my working copy a ./configure build of the previous
commit tree with the master branch of libpcap found that the function
was "missing" and tried to substitute it with the local implementation
and eventually failed trying to link with libpcap that actually had the
function.

However, this does not explain why all 32 Travis CI builds of the same
tree passed, including the builds that used autotools.