custom-command-depends-order-only
---------------------------------

* The :command:`add_custom_command` command learned the ``DEPENDS_ORDER_ONLY``
  keyword option to indicate that the command may require that custom commands
  that are part of the target using the output of the custom command may be
  necessary, but compilation of the target is not.  For example, a generated
  header that is provided by a target may be needed, but the target itself
  need not be present.
