build: fix build on Windows (patch and commit message by Adrien Nader).

CI failures and further testing made me find four more issues:
- forgot to rename a variable from "OCAMLDOC" to "WITH_OCAMLDOC"
  (this issue was also in the non-.nt Makefile)
- syntax error in the installopt rule: missing "fi"
- testsuite doesn't have Makefile.nt so don't use -f Makefile.nt for the
  "clean" rule
- had put an extra call to make "ocamltoolsopt" which in turn built the
  "opt" rule in tools/ but which isn't buildable because it is used to
  build profiling.cmx which, afaiu, is not available on windows (iirc it
  uses posix signals to "sample" the running application).
  That issue was only triggered when building "opt" and not "world.opt"
  and this is why I hadn't noticed it.

git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@13947 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02