How are compiled C++ stubs?

I’m trying to build software that use dune as build system and have C++ stubs. And compilation fails. I’ve already reported https://github.com/AltGr/ocaml-mccs/issues/12 and https://github.com/janestreet/re2/issues/21.

My understanding is that there are C++ files which must be compiled by a C++ compiler; and dune asks the OCaml compiler whether it knows about a C compiler and uses the result. Is my understanding correct? Is that supposed to work? Can I manually override this to tell dune about a C++ compiler (e.g., to trust the CXX environment variable)?