configure: --enable-headless is dead long live --without-x

To avoid confusion with the --headless runtime option
leverage --without-x for the headless build. Please note that
build and code ifdefery naming is not changed.

Change-Id: Iefdbc7ca5b4bb5627ebd2c78b4cedf44dce4ae5b
Reviewed-on: https://gerrit.libreoffice.org/12303
Reviewed-by: Michael Stahl <mstahl@redhat.com>
Tested-by: Michael Stahl <mstahl@redhat.com>