Set -o nounset in addition to +o errexit -o nounset.
    
    groups/000.host_tools.group:libtool_host:${PKG_LIBTOOL}: build w/ `none'.
    subr/build_init.subr:buildp_init_getopts(): set ${DEFAULT_BUILD_CPUS} prior to ex_pkg_load_vars() call.
    subr/pkg_{build,install_make}.subr: only pass `LIBTOOL=${PKG_LIBTOOL}' if set and not `none'.
    vars/libtool_host.vars: removes obsolete vars file.