Fixed MinGW build by declaring the minimal required Windows version.

Also deleted miscellaneous remaining multimon cruft.
Deleted #include <winuser.h>, because <windows.h> includes it
already.

This should have a nice side effect of preventing us from
accidentally using any Windows API that's too new.

modules/highgui/src/precomp.hpp

@@ -57,6 +57,14 @@
 #include <assert.h>
 
 #if defined WIN32 || defined WINCE
+    #if !defined _WIN32_WINNT
+        #ifdef HAVE_MSMF
+            #define _WIN32_WINNT 0x0600 // Windows Vista
+        #else
+            #define _WIN32_WINNT 0x0500 // Windows 2000
+        #endif
+    #endif
+
     #include <windows.h>
     #undef small
     #undef min

modules/highgui/src/window_w32.cpp

@@ -43,27 +43,11 @@
 
 #if defined WIN32 || defined _WIN32
 
-#define COMPILE_MULTIMON_STUBS // Required for multi-monitor support
-#ifndef _MULTIMON_USE_SECURE_CRT
-#  define _MULTIMON_USE_SECURE_CRT 0 // some MinGW platforms have no strncpy_s
-#endif
-
-#if defined SM_CMONITORS && !defined MONITOR_DEFAULTTONEAREST
-#  define MONITOR_DEFAULTTONULL       0x00000000
-#  define MONITOR_DEFAULTTOPRIMARY    0x00000001
-#  define MONITOR_DEFAULTTONEAREST    0x00000002
-#  define MONITORINFOF_PRIMARY        0x00000001
-#endif
-#ifndef __inout
-#  define __inout
-#endif
-
 #ifdef __GNUC__
 #  pragma GCC diagnostic ignored "-Wmissing-declarations"
 #endif
 
 #include <commctrl.h>
-#include <winuser.h>
 #include <stdlib.h>
 #include <string.h>
 #include <stdio.h>