meson.build: define NOUSER on Windows for lighter windows.h

A few exceptions are needed for sources which include COM header
(directly or indirectly).

This fixes lots of shadow warnings, see
https://github.com/MusicPlayerDaemon/MPD/issues/1150

src/fs/StandardDirectory.cxx

@@ -17,6 +17,10 @@
  * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
  */
 
+#ifdef _WIN32
+#undef NOUSER // COM needs the "MSG" typedef, and shlobj.h includes COM headers
+#endif
+
 #include "StandardDirectory.hxx"
 #include "FileSystem.hxx"
 #include "XDG.hxx"