Fixed bug 4075 - configury adds Metal.framework to linkage even if it is not available

Ozkan Sezer

Configury adds Metal.framework to linkage even if it is not available.
My solution is setting enable_render_metal to no when Metal.framework
is not found

configure.in

@@ -2093,6 +2093,8 @@ AC_HELP_STRING([--enable-render-metal], [enable the Metal render driver [[defaul
             AC_DEFINE(SDL_VIDEO_RENDER_METAL, 1, [ ])
             SOURCES="$SOURCES $srcdir/src/render/metal/*.m"
             SUMMARY_video="${SUMMARY_video} metal"
+        else
+            enable_render_metal=no
         fi
     fi
 }