configure: store $target and $host in the build configuration (fixed).
(patch by Adrien Nader!)

Compared to the previous version of this patch, this fixes too issues.

First, it updates the makefiles for mingw and msvc to set $host and
$target. I'm not completely sure why that was required since these
variables should have evaluated to empty strings which were only tested
for equality later on. Still, it's needed and it avoids an asymetry
between the regular ./configure-based builds and the ones using pre-made
Makefiles.

Second, it update the ocamlbuild config to also store $host and $target.
I had been building the cross-compilers with OCAMLBUILD_NOBOOT (which is
rather new) and updating that config wasn't needed. However it is
required for the "regular" builds.

git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@13862 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02
win32unix: add a Makefile (not ".nt" one).
(patch by Adrien Nader!)

git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@13863 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02
build: make building ocamldoc, ocamlbuild and ocamldebug optional.
(modified patch from Adrien Nader!)

Add also new comments for the new INSTALL flags.

git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@13864 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02