Ok, figured the switch is only there to check whether the "libxml2 stuff" is installed and in case it isn't it'll use the string "WITH_XML="--disable-libxml …"
In my case I uninstalled the libxml2-dev and then reinstalled it again with
apt-get install libxml2-dev
and for the sake of it, a reboot on top. After this I reinstalled ISPConf 2.2.27 out of the box — no mods anywhere! — and finally got the "libXML support active" showing in the