Support libev as a libevent alternative

[rdmark]

libev claims to be a better and faster libevent, with a libevent emulation layer

However Meson doesn't detect it as such right now, so the dependency check for libevent needs modification at the very least.

http://software.schmorp.de/pkg/libev.html

[rdmark]

The relevant Debian dev package is libev-libevent-dev

[rdmark]

We're blocked by a lack of a event_base data structure and event_new() callback support in libev.