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

[Jerome Guitton]

Eli Zaretskii (eliz@gnu.org):

> > Same result: case sensitive.
> 
> Thanks, now we know.

Hmm. From what I read in Cygwin's documentation, the situation could
actually be a bit more complex than that. There is at least an option
(upcaseenv) that can have an impact on Cygwin's behavior. So a test at
configure time would be useful.