Lowercase windows imports

While windows and macOS are currently on case-insensitive file
systems, this poses no issue on those.  When cross compiling from
linux with a case sensitive file system and mingw providing only
lowercase headers, this in fact produces an issue.  As such we just
lowercase the import headers, which should still work fine on a
case insensitive file system and also enable mingw's headers to
be usable porperly.