As per http://www.thinkwiki.org/wiki/How_to_configure_the_TrackPoint#Configuration_using_sysfs, on my X200 (which has no touchpad) with Ubuntu 16.04, I used sudu -i to become superuser, then echo -n 1 > /sys/devices/platform/i8042/serio1/press_to_select to turn it on and now it works, though it's a little more sensitive than I might like so it'll take some getting used to. Thinkpads with a touchpad will have a slightly different path in the command, see the link. Also in the link is info on making the change permanent, if desired.
Edit: Found the related https://askubuntu.com/a/761504/392539, which happens to include a possibly simpler way to make this permanent (along with other trackpoint settings), but haven't tried it myself.