OCaml bindings: fix: only pass -fpermissive when compiling C++

OCaml bindings: fix: only pass -fpermissive when compiling C++

The `-fpermissive` flag is only valid for Objective-C and C++, but was being set
for C and C++. When compiling the C part of these bindings, this was generating
the warning:

  cc1: warning: command line option ‘-fpermissive’ is valid for C++/ObjC++ but
    not for C

This flag was introduced in 8f4667edb410a6d11b53746849304fb953b5c6ae to work
around non-conformant code generation by SWIG. In future, it might be worth
investigating whether the underlying issue has been fixed in SWIG and this work
around can be removed.