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
config/Makefile.{mingw,msvc}{,64}: remove unused DO variable.

(Patch by Adrien Nader!)

This variable is currently unused and it breaks my new script which
replaces mkmyocamlbuild.sh.
The issue is that the "DO" variables becomes "do" in
myocamlbuild_config.ml (lowercased) and "do" is an OCaml keyword.
Since the variable is not used, simply remove it.

Also remove the DBGO variable which only exists in Makefile.msvc64.
Checking for occurrences of this variable elsewhere, I only found
byterun/Makefile.nt which defines it as DBGO=d.$(O) rather than
dbg.$(O).

git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@14163 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02
build: allow $(UNIXDIR) to be "win32unix".

(Patch by Adrien Nader!)

Several commits ago, I made a change that replaced includes for
"otherlibs/unix" with "otherlibs/$(UNIXLIB)", making it possible to
include "win32unix".
Apparently I forgot to do it for debugger/Makefile or skipped it on
purpose since it only matters for cross-compilation and cross-compiling
the debugger is not supported (yet).
Change it now rather than forget it later.

git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@14164 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02