ssss: modernize

The CC replacement was invalid, and superenv removes
the need for setting include paths manually.
arp-sk: use libnet prefix
Mercurial 3.1

Version bump from 3.0.2 to 3.1. I’ve also changed the mirror because
the current mirror doesn’t have the least 3.1 package up yet.
mercurial: update 3.1 bottle.