APP_ICON populates the following CMake variables (if undefined):

CPACK_INNOSETUP_ICON_FILE = ${_ico}
CPACK_INNOSETUP_SETUP_WizardSmallImageFile = ${_ico}
CPACK_PACKAGE_ICON = ${_ico}
CPACK_WIX_PRODUCT_ICON = ${_ico}
CPACK_WIX_UI_BANNER = ${_ico}
CPACK_WIX_UI_DIALOG = ${_ico}