Remove MSVC scripts from the tree

Hi all,

It has been mentioned a few times now that, as Meson has been
integrated in the tree, the next step would be to get rid of the
custom scripts in src/tools/msvc/ and moving forward only support
Meson when building with VS compilers.  As far as I understand, nobody
has sent a patch to do that yet, so here is one.

Please find attached a patch to move the needle in this sense.  Here
are some notes:
- Meson depends on msvc/gendef.pl, so I've renamed it to
msvc_gendef.pl in src/tools/.
- install-windows.sgml could be OK if entirely gone, moving more
detailed instructions to the meson page instead.
- What to do with src/tools/msvc/dummylib/?  It would still be useful
for src/tools/win32tzlist.pl but it also seems to me that we should be
able to live without it as perl's Win32 should have evolved quite a
bit?  I need to test this, but I'd like to think that we are OK with a
removal of it.  If people want to keep it.  I'm OK with that as well.

The documentation for Meson could be improved more, I think, when
building with either MinGW's or VS compilers.  Particularly, something
that was itching me is how we can improve the instructions about the
extra packages one would need to deploy to make the builds work as a
straight removal of install-windows.sgml loses references, but we are
OK if we rely on a packager to do the dependency job.  For example,
I've been having a good ride with Strawberry Perl and Chocolatey,
linking my build of Meson with MinGW or Visual, but I also recall
Andres being allergic to Strawberry, so I am not sure if folks are OK
with directly mentioning it in the docs, for instance.

One thing that we could do is add a subsection in the installation
notes close to MinGW, but for Visual Studio that applies to Meson.
One good thing with the removal of install-windows.sgml is that it is
possible to clean up a few things that have rotted in the docs, and
could be reworked from scratch.  For example, we still recommend
Active Perl, but it has become impossible to use in builds as perl
commands cannot be involved without extra steps that we don't
recommend, while an installation needs to be registered in their
centralized system..  That's not user-friendly.  I've notice that
Andrew has been relying on strawberry perl as well with Chocolatey for
some buildfarm members.

As of today, I can see that the only buildfarm members relying on
these scripts are bowerbird and hamerkop, so these two would fail if
the patch attached were to be applied today.  I am adding Andrew
D. and the maintainers of hamerkop (SRA-OSS) in CC for comments.

Thoughts?
--
Michael