Enable to read which backend was picked by libev.

[mefyl]

My use case for this is letting libev choose its backend freely, but checking it did not fallback to select in production as the 1024 file descriptor limit is not fit for purpose.

[mefyl]

I'm at a loss as to why this fails in ocaml-ci with undefined reference to 'lwt_libev_backend' on all platforms; it absolutely works locally for me and the function is present in the C stub file 🤔

[raphael-proust]

This seems like a reasonable addition. I'll look into the failure.

[raphael-proust]

I started the other CI tests (I just noticed they needed an approval start) and they paint a much clearer picture: the error occurs when the code is compiled with libev deactivated.

I've pushed a commit adding some lazy because I believe the error occurs as a result of initialisation when libev is absent.

[raphael-proust]

It doesn't work, same error. I'll dig further.

[bnguyenvanyen]

Hi, we're still interested in having this upstream.
I don't know how to reproduce the CI failure, but I made a suggestion for the fix in the code: Use the LWT_NOT_AVAILABLE1 macro when libev is not available, similarly to the other functions in the module.

[mefyl]

The remaining CI issues seem to be the same as master AFAICS.

[raphael-proust]

Thanks!
I'm hoping to do a 5.8.1 release soon and I'll include this. (Sorry I only noticed your changes after pushing the 5.8.0 through.

[raphael-proust]

Thanks a lot for the contribution <3