Thank you for the proposed patch. I tried using it, and it appears that unison can't find the fsmonitor helper when it's named "unison-$(ABI)-fsmonitor". Instead it should be named "unison-fsmonitor-$(ABI)". See <https://github.com/bcpierce00/unison/blob/ce6f66b048724687d716df33d5b47a4a5634b936/src/fswatch.ml#L268>
I think you need something like the attached patch (disclaimer: works on my machine, that's as much testing as I did).
thank you. I applied your patch on top, it works for me too.
Regards