windows: Added WIN_IsWindowsXP, for extreme runtime compatibility checks.

Reference Issue #8666.

src/core/windows/SDL_windows.h

@@ -145,6 +145,9 @@ extern void WIN_CoUninitialize(void);
 extern HRESULT WIN_RoInitialize(void);
 extern void WIN_RoUninitialize(void);
 
+/* Returns SDL_TRUE if we're running on Windows XP (any service pack). DOES NOT CHECK XP "OR GREATER"! */
+extern BOOL WIN_IsWindowsXP(void);
+
 /* Returns SDL_TRUE if we're running on Windows Vista and newer */
 extern BOOL WIN_IsWindowsVistaOrGreater(void);