First I ran
setxkbmap -option ctrl:swapcaps in gnome-terminal, and observed that the keys were switched on my Macbook keyboard.
Now, following an answer on the web whose URL I've lost, I added that line of text to
Note that the
setxkbmap line the worked in Ubuntu 16.04,
setxkbmap -option caps:ctrl_modifier, did not work immediately when run in terminal in Lubuntu.