Which you do with Configure -Dversiononly. (That has the side effect of clearing inc_version_list/inc_version_list_init, so you may want to set those in config.sh after configuring and then rerun Configurure -S or config_h.SH if you are trying to have e.g. 5.8.2 and 5.8.5 separate, but have 5.8.5 use 5.8.2's installed modules. This is not an issue for using both 5.6.x and 5.8.x.)