--config-dir option in configure script

Adds a `--config-dir` option to the `configure` script to provide a way of changing the default system wide config file prefix (default is `/etc`).
![image](https://f.cloud.github.com/assets/353843/568915/e7bd4e72-c6e9-11e2-8c35-580f469eb349.png)
Will be useful for creating a [homebrew](http://brew.sh) formula to help osx people build the latest version from git sources.