Fix f_system on Mac OS X

Before:
![before](https://f.cloud.github.com/assets/11351/2059620/9aaea8cc-8bc1-11e3-9607-91df2f5661e2.png)

After:
![after](https://f.cloud.github.com/assets/11351/2059621/9e936e3c-8bc1-11e3-9fdb-dd9ab434d6e6.png)

System Info:
![matches](https://f.cloud.github.com/assets/11351/2059622/a5d86256-8bc1-11e3-912b-c8d8e20380cb.png)