Further refine the C2C.atom_is_external criterion in "no-common" mode

In `-fno-common` mode, a global variable declaration such as `int x;`,
with default storage class, acts like a definition: `x` cannot be
defined in a DLL.

cfrontend/C2C.ml

@@ -57,6 +57,7 @@ let atom_is_external a =
   match Hashtbl.find decl_atom a with
   | { a_defined = true } -> false
   | { a_storage = C.Storage_static } -> false
+  | { a_storage = C.Storage_default; a_size = Some _ } -> !Clflags.option_fcommon
   | _ -> true
   | exception Not_found -> true