rb_sys_fail(must enable kqueue) on OSX Lion

Similar to eventmachine/eventmachine#39

```
/Library/Ruby/Gems/1.8/gems/eventmachine-0.12.10/lib/eventmachine.rb:1266: [BUG] rb_sys_fail(must enable kqueue) - errno == 0
ruby 1.8.7 (2010-01-10 patchlevel 249) [universal-darwin11.0]
```
