added support for '--etcdir' in 'configure' (default: '/etc')

 * for backwards-compatibility, the default is `'/etc'`, which causes the
    config files to go in `'/etc/elvis'`;
 * this feature was added to allow non-"distro" controlled directories
    to be used (for example, in the user's `'$HOME'` directory, or a
    directory under `'/usr/local'`), which improves separation between
    "experimental" or "manually built" installations and
    "distro-controlled" or "standard" ones;

As an example, you can *"configure"* elvis to use an `'etc'` directory within its install directory:

```sh
$ ( p=/usr/local/opt/elvis-2.2_git_01_x11 && ./configure --verbose --with-x --x-libraries=/usr/lib64 --prefix=$p --etcdir=$p/etc linux ) ; echo "rc: $?"
```