[RFA/mingw32] environment variables are case-insensitive on win32

[Eli Zaretskii]

> From: Jerome Guitton <guitton@adacore.com>
> Cc: Jerome Guitton <guitton@adacore.com>
> Date: Fri, 30 Nov 2012 14:36:37 +0100
> 
> A patch that I sent a few months ago, I think, but never got in:
> environment variables are case-insensitive on windows, this patch would
> take that into account.

Thanks.  But why did you exclude Cygwin?

> It would have been nice to have a configure test for that, using
> setenv/getenv to detect the case insensitivity.  Unfortunately we
> don't have setenv on win32. So I ended up using _WIN32 just like we do
> to handle the .exe extension. Any other idea?

Use putenv?