Fix build problems due to "missing" newer MSW APIs with MinGW 5.3.0

Set WINVER/_WIN32_WINNT ourselves before letting MinGW to set them to very low
values corresponding to Windows 2000 on its own and preventing our code from
seeing any later additions to the Windows API, such as AttachConsole()
function used in src/msw/app.cpp.

Closes #17677.

include/wx/msw/gccpriv.h

@@ -33,6 +33,14 @@
         #endif
     #endif
 
+    /*
+        MinGW 5.3.0 (and presumably later) predefines _WIN32_WINNT and WINVER
+        in sdkddkver.h included from _mingw.h to very low (Windows 2000!)
+        values. We really want to predefine them ourselves instead, so do it
+        before including _mingw.h.
+     */
+    #include "wx/msw/winver.h"
+
     #include <_mingw.h>
 
     /*