Update WINVER to 0x500 to avoid build failures from Jacob's

FlashWindow changes. Also fiddle with the <multimon.h> include,
which was subtly broken in turn by that.

[originally from svn r7120]

windows/window.c

@@ -10,6 +10,10 @@
 #include <limits.h>
 #include <assert.h>
 
+#ifndef NO_MULTIMON
+#define COMPILE_MULTIMON_STUBS
+#endif
+
 #define PUTTY_DO_GLOBALS	       /* actually _define_ globals */
 #include "putty.h"
 #include "terminal.h"
@@ -17,11 +21,8 @@
 #include "win_res.h"
 
 #ifndef NO_MULTIMON
-#if WINVER < 0x0500
-#define COMPILE_MULTIMON_STUBS
 #include <multimon.h>
 #endif
-#endif
 
 #include <imm.h>
 #include <commctrl.h>