Good point, thank you (and even cl.exe, Visual Studio compiler supports -E)! I believe a simple solution is then to use cc -E to preprocess the .ml file directly.
Well, and FreeBSD 
… “Unix” has been lost in this conversation. This last version broke things for us on FreeBSD. I’ll have a PR for you asap.
Sorry for that! Thank you in advance for the PR, I hope to make a release next week.