Most probably this means you already have it installed, otherwise slackpkg would have detected and installed it. Just in case, check if you have it installed (don't forget the "p" after lib)
slackpkg search libpcap
If it's installed, try installing the tcpdump package, which contains the pcap headers, and try again to build.
If this still doesn't help, check out this thread
for a solution. It seems that the library and its devel version have been a little bit messed up at that time. No idea if it's fixed by now, but it's worth a try.