As an update, cloning the cron repo and then copying in all the files referenced by the Makefile (a la carte from the mjs, mongoose-os, and mongoose repos) does indeed produce a compilable and runnable version of ./cron_test in the test folder of cron. (Here is a link to my fork if you would like to see exactly what I did.)
Unfortunately the tests do not actually pass, but since that is a different issue entirely, I have created a new thread for it, and the main topic of this thread has been resolved.