macro(itk_set_with_default var value)
  if(NOT ${var})
    set(${var} "${value}")
  endif()
endmacro()

# Bridge an old, deprecated, setting to a new replacement setting.
#
# Use this function when a user-visible flag is being renamed or otherwise
# replaced. If the old value is set, it will be given as the default value,
# otherwise the given default value will be used. This returned value should
# then be used in the ``set(CACHE)`` or ``option()`` call for the new value.
#
# If the old value is set, it will warn that it is deprecated for the new name.
#
# If replacing the setting ``OLD_SETTING`` with ``NEW_SETTING``, its usage
# would look like:
#
#   itk_deprecated_setting(default_setting NEW_SETTING OLD_SETTING "default value")
#   set(NEW_SETTING "${default_setting}"
#     CACHE STRING "Documentation for the setting.")
function(
  itk_deprecated_setting
  output_default
  new
  old
  intended_default)
  set(default "${intended_default}")
  if(DEFINED "${old}")
    message(WARNING "The '${old}' variable is deprecated for '${new}'.")
    set(default "${${old}}")
  endif()

  set("${output_default}"
      "${default}"
      PARENT_SCOPE)
endfunction()