PR#6075: fix build under Windows (there is no caml_strbuf_free).

git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@14651 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02
#6273: fix Sys.file_exists on large files for Win32 (patch by C. Bauer).

git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@14652 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02
Merge remote-tracking branch 'ocaml/trunk' into trunk-gdb