configure: renamed '--etcdir' -> '--sysconfdir'; Makefile.in: reverted unrelated fix;

 * we now use '--sysconfdir', for greater consistency with other
   'configure' scripts (as per github comment by @ib);
  * renamed (non-user-visible) variables for consistency;

 * removed a fix (to be committed to another topic branch) which made it
    possible to use a '--prefix' directory that does not exist already
    (as per github comment by @ib);