[lmi] Fixing libxslt build with MinGW 10 and using submodules for third-party libraries

[Vadim Zeitlin]

 Hello yet again,

 While testing building of lmi with MinGW-w64 10.1 we've run into problems
when building libxslt. One of these problems has been already fixed by the
upstream, while the other one hasn't been, so we have the choice between
updating the latest release including the first fix and applying our patch
for the second fix on top of it, or just applying both patches on top of
the current version.

 I can, and will, of course, provide patches implementing the solution you
prefer once you will let me know which one this would be and I'd also like
to understand how is it possible that you haven't run into these problems
yourself.

 However beyond this, I'd like to return to my proposal to use Git
submodules for the third party libraries instead of downloading them via
FTP and patching them manually. Our last discussion about this started at
https://lists.nongnu.org/archive/html/lmi/2019-09/msg00013.html seems to
have petered out without really reaching any conclusion, in spite of the
initially positive reception. Rereading it, it looks that the main reason
for this might have been the fear of breaking something before the major
Halloween release of lmi. Of course, by now we're close to another
Halloween, but I am not sure if this means that another major release is
planned for it?

 If not, I'd really like to at least experiment with using submodules.
Concretely, right now I'd like to add third_party/libxslt submodule and
change install_libxml2_libxslt.make to use it.

 Should I go ahead with this?
VZ

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