hook_preconfigure()

Here's a fairly complicated example (this is from GCC):

# The "target" variable is the compilation target: "ntoaarch64le", "ntox86_64", etc.
function hook_preconfigure {
    case ${SYSNAME} in
    nto)
        case "${target}" in
        nto*)   basedir=/usr ;;
        *)      basedir=/opt/QNXsdk/host/qnx/x86_64/usr ;;
        esac
        ;;
    linux)
        host_cpu=$(uname -p)
        case ${host_cpu} in
        i[34567]86) host_cpu=x86_64 ;;
        esac
        basedir=/opt/QNXsdk/host/linux/${host_cpu}/usr
        ;;
    *)
        echo "Don't have config for ${SYSNAME}"
        exit 1
        ;;
    esac
    configure_opts="${configure_opts} --target=${target}"
    configure_opts="${configure_opts} --prefix=${basedir}"
    configure_opts="${configure_opts} --exec-prefix=${basedir}"
    configure_opts="${configure_opts} --with-local-prefix=${basedir}"
    configure_opts="${configure_opts} --enable-haifa"
    configure_opts="${configure_opts} --enable-languages=c++"
    configure_opts="${configure_opts} --enable-threads=posix"
    configure_opts="${configure_opts} --with-gnu-as"
    configure_opts="${configure_opts} --with-gnu-ld"
    configure_opts="${configure_opts} --with-as=${basedir}/bin/${target}-as"
    configure_opts="${configure_opts} --with-ld=${basedir}/bin/${target}-ld"
    if [ ${SYSNAME} == nto ]; then
        configure_opts="${configure_opts} --enable-multilib"
        configure_opts="${configure_opts} --enable-shared"
    else
        configure_opts="${configure_opts} --disable-multilib"
    fi
}