[5.2 & 5.3] Compiling C++ code using the OCaml C API results in name-mangled caml_state on Cygwin

Trying to compile opam (which uses C++ through mccs) on Cygwin with the 5.3 branch at 162a7f4869bb3136b48fd811408cc6f53396a0ea, fails with:
```
  ** Cannot resolve symbols for descriptor object:
   _ZTH10caml_state
  File "caml_startup", line 1:
  Error: Error during linking (exit code 2)
```
Whereas it compiles just fine with OCaml 4.14.2 with the same environment.