win32: define _WIN32_WINNT globally

Rather than defining _WIN32_WINNT only in a handful of files, define
it in config.h, like we do with _GNU_SOURCE.

(Also remove a "#define WIN32_LEAN_AND_MEAN" that isn't really all
that useful.)

https://bugzilla.gnome.org/show_bug.cgi?id=688109

gio/win32/gwin32directorymonitor.c

@@ -21,8 +21,6 @@
  * 
  */
 
-#define _WIN32_WINNT 0x0400
-
 #include "config.h"
 #include "gwin32directorymonitor.h"
 #include <windows.h>