Avoid multiple declarations in generated .c files in -output-obj

Avoid multiple declarations in generated .c files in -output-obj

In -output-obj mode, <caml/mlvalues.h> (which contains some
primitives) is included in the generated .c file, leading to errors
when compiling with g++ (multiple declarations).

There are probably better implementations (in particular, in this one,
care must be taken when changing the list of primitives available in
mlvalues.h), but this is a small and (not too) intrusive patch.

Bug: http://caml.inria.fr/mantis/view.php?id=5254
Signed-off-by: Stephane Glondu <steph@glondu.net>
Gbp-Pq: Name 0005-Avoid-multiple-declarations-in-generated-.c-files-in.patch