Add builder to build tor with libevent 1.4
Weasel tells me these are the steps.
- https://gitweb.torproject.org/mirror/libevent.git/ needs to be up-to-date and it should have a 1.4 branch.
- Collect commands and build dependencies to build libevent 1.4; have weasel make a builder.
- Figure out how to build Tor against that libevent.