PR#6084: Define caml_modify and caml_initialize as weak symbols to help
with Netmulticore.  (Reflecting commit r13957 on version/4.01)


git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@13958 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02