Okay, update time! tl;dr -- I got it working! I managed to compile the 0.80.0 version so it looks like that functionality is indeed not included in the current (0.70.6) version that is available to download at itch.io as of Feb 21, 2019
I'm not entirely certain what caused it to work but my bet is currently that I had initially cloned the repository incorrectly: I hadn't used the --recursive flag so I removed the folder and re-cloned the project, and after that cmake . "just worked" and now I have version 0.80.0 and when I save a .fnl file it does, in fact, save a .fnl file (and not a .fnl.tic file). So now I'm looking forward to doing some Fennel coding! :-D