[lmi] Updating install_wx.make to get wxWidgets from git

[Vadim Zeitlin]

 Hello,

 I'd like to finally make the long-promised patch implementing getting
wxWidgets from Git repository, instead of using GitHub snapshots, which
don't include submodules, but I realized that I'm not sure what would be
the best way to do it. The question is whether we should optimize for the
"once only" case or for repeated invocations of install_wx.make (with
either the same or different wx_commit_sha values)? In the latter case
(which is mine), it would make sense to do "git clone" once, if the
directory doesn't exist, or just "git clean" + "git fetch" if it does
already exist to significantly speed up the second and subsequent
executions, but at the price of making the first run slower as the full
clone takes more time than a shallow clone retrieving just the given commit
(although it's difficult to say by how much exactly, without doing several
time-consuming tests).

 Would you have any preferences here or should I just do the simplest, and
most convenient for me personally, thing and use "git clone" to make
updating faster?

 Thanks in advance,
VZ