The singular name `CPACK_INSTALL_SCRIPT` has existed but was not linked
from the CPack documentation. Also, it supported multiple values and
should have had a plural name. Add a plural-named alternative now.
If both `CPACK_INSTALL_SCRIPTS` and `CPACK_INSTALL_SCRIPT` are set then
ignore the latter with a warning.
Signed-off-by: Alex Turbov <i.zaufi@gmail.com>
