libpcap: pass --with-pcap=linux for all linux systems

...instead of just for i686-linux.

Without a --with-pcap= configure flag, the build system tries to
auto-detect the backend by poking in /dev, /usr/include etc... In the
pure nix world, this auto-detection fails. (Good, we don't want host
details to leak into the build.)

The end result of this (failed) auto-detection; no packet capture
possible:

  $ sudo tcpdump -i wlp2s0
  tcpdump: live packet capture not supported on this system

This fixes the above issue on 64-bit linux systems, and probably other
architectures supported by linux.