Fix detection of system libnet.

The latest version of libnet from https://github.com/sam-github/libnet has `#define LIBNET_VERSION  "1.2-rc2"`. This causes the current version to not detect the presence of the system libnet because the regex used to extract the version number only includes numbers and periods.

This fixes the detection of 1.2-rc* libnet versions. No problems exist when linking to the newer libnet library to the best of my knowledge.