Fix trace compilation for old ocaml.

[jpoiret]

The Ast change with Pexp_function occured in Ocaml 5.2.0 IIUC, so the optional compilation needs to be based on the ocaml version, not ppxlib's.

[gares]

According to opam ppxlib 0.36 works with ocaml 4.8 to OCaml 5.3 included. Internally it is able to map one AST to many.
And I'm sure that with OCaml 4.14 and ppxlib 0.36 Elpi would not compile because of the AST change.
My understanding is that ppxlib 0.36 started to use OCaml's 5.2 AST as the input one. But that is unrelated to the version of OCaml you are actually using to compile Elpi.

So I'm a bit confused about your change. Which configuration did you try and did not work?

[jpoiret]

Ah no you're completely right, there's a complicated configuration issue on my machine that misreported the ppxlib version and led to the conditions not working as intended. Sorry for just sending this wrong PR (it did resolve the issue locally but for the wrong reasons).