OCaml 4.10 support

[kit-ty-kate]

The beta release of OCaml 4.10 is around the corner, and currently, the latest version of ocurl does not support 4.10 and fails with 900 similar lines of the following error message:

- /home/opam/.opam/4.10.0+trunk/lib/ocaml/caml/compatibility.h:32:26: error: expected identifier before '(' token
-    32 | #define caml_local_roots (Caml_state_field(local_roots))
-       |                          ^
- /home/opam/.opam/4.10.0+trunk/lib/ocaml/caml/domain_state.h:53:45: note: in definition of macro 'Caml_state_field'
-    53 | #define Caml_state_field(field) Caml_state->field
-       |                                             ^~~~~
- /home/opam/.opam/4.10.0+trunk/lib/ocaml/caml/compatibility.h:279:21: note: in expansion of macro 'caml_local_roots'
-   279 | #define local_roots caml_local_roots
-       |                     ^~~~~~~~~~~~~~~~

[ygrek]

Thanks for the report