fix compilation under FreeBSD

Trying to compile under FreeBSD throws this error.
![screen shot 2014-02-14 at 3 38 49 pm](https://f.cloud.github.com/assets/1450169/2176350/2f9a68ea-95d1-11e3-9201-23187b9a144c.png)

I fixed this by not defining `_XOPEN_SORCE` only in FreeBSD.