Build on OCaml 5 (ocamlnet -safe-string workaround)

[aantron]

With this PR, it is possible to build and run OCaml.org on OCaml 5 with

make deps
make start

The build was failing due to problems with a configuration script in ocamlnet, a transitive dependency of OCaml.org, through package River. For details on the problem in ocamlnet, see https://github.com/aantron/ocamlnet/commit/7ef255a0a994ef2721f3359077b36bcdd0428ee4:

Work around broken HAVE_BYTES detection

configure uses ocamlc -safe-string to determine whether the compiler has the bytes type. However, on OCaml 5, this command, without input files, exits with a non-zero exit code and prints No input files. This causes configure to wrongly believe that -safe-string is not supported, which then triggers a multitude of compilation errors.

This commit (in ocamlnet) changes the code to unconditionally assume that bytes is available. A better way is probably to adapt OASIS, or what generated the configure script.

The code in ocamlnet's configure script is here. cc @gerdstolpmann

This PR pins ocamlnet to a fork with the above workaround, until ocamlnet is fixed upstream and released. See the OCaml 5 compatibility MR in ocamlnet.

This PR points package ocamlnet to my own fork on GitHub, which could become a problem for git bisect or other purposes in the future, if the fork were to become inaccessible. However, I see that OCaml.org already has dependencies pointed to other repos, so perhaps this is fine. Please let me know if you'd like me to put the ocamlnet patch somewhere else.