I think relying on shell scripts to create a random string sequence just to append to backup filenames is a bit fragile one you need it to work across different platforms. At the very least, it should be taken out of the makefile so you can run the other commands without throwing errors.
On OS X running GNU Make 3.81 (and the BSD version of tr I suppose?) I'm getting the error
which happens even when I try to run this bit independently
https://github.com/BirkbeckCTP/janeway/blob/bbd426f2b5b8da40d2674a51c25b3aa8ad8d3f7b/Makefile#L41
I think relying on shell scripts to create a random string sequence just to append to backup filenames is a bit fragile one you need it to work across different platforms. At the very least, it should be taken out of the makefile so you can run the other commands without throwing errors.