AC_ARG_WITH(firewall,
[  --with-firewall=< pf | ipfw | iptables >
                          Select the firewall backend],
[FWALLSDIR="/my/path"
case "${withval}" in
     pf)
        cp $(FWALLSDIR)/command_pf.h $(FWALLSDIR)/command.h
        usepf=true
        ;;
     ipfw)      useipfw=true ;;
     iptables)
        cp $(FWALLSDIR)/command_iptables.h $(FWALLSDIR)/command.h
        useiptables=true
        ;;
     *)
        echo "Choose a valid firewall backend (see --help)"
        exit 1
        ;;
    esac], AC_MSG_ERROR([Error: please choose a valid firewall backend (see --help)])
    )