build: ocamldoc.opt was not built anymore on Windows.
(Patch by Adrien Nader!)
An earlier change I had made conditionalized building ocamldoc.
I mixed up my variable names and set "WITH_OCAMLDOC_OPT" instead of
"OCAMLDOC_OPT". Later on, $(OCAMLDOC_OPT) would evaluate to the
empty-string, failing to add "ocamldoc.opt" to the prerequesites of the
"opt.opt" and "world.opt" rules.
git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@14162 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02