[lmi] wxWidgets submodules and install_wx.make

[Vadim Zeitlin]

 Hello,

 As alluded to in a previous message, there is a problem with using
install_wx.make with the latest versions of wxWidgets because downloading
wxWidgets archive doesn't include the submodules and even though lmi
doesn't need all of them (and so anybody working on it would profit from
having a smaller local working copy not including libjpeg or libtiff), it
does need some of them (expat, libpng, zlib).

 I'm not sure what would be your preferred solution to this problem. At the
top level of the decision tree is the question of whether lmi needs to
continue to use 3rd party libraries bundled with wxWidgets[*] or if it can
use them directly. IMO it's not impossible that using some official builds
provided by MinGW community could be a better solution, in theory, but I'm
not sure if we want to do it right now. So I'll assume we do want to
continue to use the versions bundled with wxWidgets, but please correct me
if I'm wrong.

 If we do this, the only solution I see is to download all of the archives
from the corresponding submodule repositories, e.g.
https://github.com/wxWidgets/libexpat/tree/wx, and unzip them in the right
place. This shouldn't be difficult to do, but it's not completely trivial
to do it using shell fragments in a makefile neither, so I wonder if we
really want to do this or if we could just use "git clone" instead? With
"--depth=1", it shouldn't take that much longer than downloading a ZIP (but
I didn't measure this), but we could use --recurse-submodules option to get
all the submodules for us. And, of course, using Git would make things
simpler for me when testing lmi with newer wxWidgets versions too, which is
nice even if it's not, and shouldn't be, a preponderant factor in the
decision.

 Of course, all this is only really a problem when using arbitrary
wxWidgets commits and wouldn't be such a problem if we used only the
official releases, which will continue to include submodules in them. But I
think it could be useful to continue to allow using non-released versions
of wxWidgets, even if it's less simple now than it was before.

 Please let me know if you have any thoughts about this,
VZ

[*] Which is, of course, itself a 3rd party library, so maybe I should
    refer to the other ones as being 9th party libraries for clarity?