fix #333

Ignore-this: 36ff363c3c97fbf01762bee2eddd7853

darcs-hash:20130315203929-c41ad-6e4ac7015f34043581dd082eadef7d599649e02f.gz
use libev by default only on linux

Ignore-this: 341053a8fa6bf7ef9a2c20a52400deb

darcs-hash:20130316110233-c41ad-3bb904e0d2d2e503061c6a994fd2e8e2541340fe.gz