Fix issue 6406

This patch add the OCAML_VERSION in the C header as integer (patch propulsed by OCaml Users in PariS and his hacking session) - http://caml.inria.fr/mantis/view.php?id=6406.
Maybe `#define OCALM_VERSION (($major << 16) | ($minor << 8) | $patchlvl)`?

The problem with just concatenating the digits is that 1.2.34 and 12.3.4 stringify to the same value.  (Another minor issue is that 0.1.2 becomes 012 or 10 in octal.  Not fatal but probably not the intended behavior either.)