Libnl3 support

libpcap currently only links libnl1 or libnl2.  Unfortunately, libnl3 is called libnl-3.so.0, so there is no versioning conflict when linking.

This means tools like Kismet which use libnl directly can link nl3, and pcap links nl1, and it all blows up when libs are loaded (instant segv).  It's also opaque at build time if pcap links nl1.

Patch adds nl3 support and breaks the nl build process into features not build numbers (since presumably nl4 will also break things again, differently).

In a perfect world we'd use pkgconfig to find the deeper depends of nl3, this just uses the nl2 method of assuming genl is required.

Since autoconf is such a beast I only included the changes to configure.in here.