clang 5.0.1 complains:

  src/systemd/src/libsystemd-network/dhcp6-option.c:40:28: error:
      field 'option' with variable sized type 'struct DHCP6Option' not at
      the end of a struct or class is a GNU extension
      [-Werror,-Wgnu-variable-sized-type-not-at-end]
        struct DHCP6Option option;
                           ^

systemd disables this warning too.