Fix autodetection of -opaque and -g

[stedolan]

The autodetection of whether -opaque and -g are supported by the compiler is broken since OCaml 5.2 (as debugged by @emillon in issue #78), because now ocamlc -opaque gives an error even though the -opaque flag is supported.

This patch changes the tested command to be ocamlc -opaque -version (which succeeds if -opaque is supported and fails if not, even on recent OCaml), and likewise for -g.