configure: fixed X11 libraries detection on RedHat/CentOS
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;