need to default to the libevent backend or some backend on CentOS 5.8

When doing the original backend work, we'd discussed and decided, I think, that the libevent backend should be the default and dependencies should follow, but what seems to be happening instead is libev is installed by default.

In at least one case it didn't work at all, and only the frontend package was installed.

This should be resolved to be easier from 2.1 forward.