add configure detection of mkstemp, remove old HAVE_MKSTEMP conditional, rename HAVE_MKSTEMP -> HAS_MKSTEMP

git-svn-id: http://caml.inria.fr/svn/ocaml/version/4.02@14929 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02
update instructions for 32-bit builds on Mac OS X (PR 6196)

git-svn-id: http://caml.inria.fr/svn/ocaml/version/4.02@14931 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02