Use SFML 2.6's set of compiler warnings

Some minor tweaks were made to remove Android-specific workarounds
that CSFML doesn't need and removing a few warnings that are not
feasible to fix at the moment.

src/SFML/Graphics/RenderWindow.cpp

@@ -347,7 +347,7 @@ sfWindowHandle sfRenderWindow_getSystemHandle(const sfRenderWindow* renderWindow
 {
     CSFML_CHECK_RETURN(renderWindow, 0);
 
-    return (sfWindowHandle)renderWindow->This.getSystemHandle();
+    return static_cast<sfWindowHandle>(renderWindow->This.getSystemHandle());
 }