Fix for FreeBSD

Fixes : https://github.com/ingydotnet/file-share-pm/issues/5

On FreeBSD, system perl modules are located in : 

```
/usr/local/lib/perl5/site_perl/5.16
```

The fact that path contains "lib" makes "dist_dir()" return : 

```
/usr/local/share
```

...which is wrong.