[lmi] [PATCH] Use submodules for, and newer versions of, libxml2 and libxslt

[Vadim Zeitlin]

 Hello,

 Here is, finally, the promised patch:

        https://github.com/vadz/lmi/pull/153

 Please note that you will need to run

        $ git submodule update --init

after merging it to actually initialize the submodules unless you rerun
install_msw.sh, which was updated to do it.

 As discussed before, I've upgraded libxslt to a custom version, based on
the latest libxslt release, but with a few extra commits we need to fix
various MinGW problems. Hopefully when the next release is done, we could
update to it and use it without changes, but, of course, one of the big
advantages of using a submodule is that it doesn't really matter from the
parent repository point of view. BTW, I did not upgrade libxml2, although
we're a few release, and ~500 commits, behind the latest master, just to
avoid changing too many things at once, but if you'd prefer, we could
update it too -- again, this is easy to do now and can be done at any time.

 Anyhow, with the current changes, I can build everything successfully
(although with some harmless warnings during libxslt build, which are
unfortunately not that simple to get rid of) using MinGW 10.1 under Debian
Sid. I haven't yet tested the full build with MinGW 8.3 under Buster, but
at least install_libxml2_libxslt.sh does work with it and I'll do more
testing tomorrow.

 I also still need to update the CI workflow file, but this shouldn't
prevent you from testing this, so I've decided to submit the PR already.
AFAIK, it can be merged, i.e. even though more changes are possible and
desirable, they can be done independently later.

 Sorry again this taking so long -- as typical, I've wasted a lot of time
trying to get the quoting right in the shell script and, finally, just did
it in the simplest possible way, without trying to preserve the old
makefile structure as much as possible.

 And, of course, please let me know if you run into any problems or have
any questions about these changes!
VZ

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