Enable building the Metal renderer by default, and weak link the Metal framework so the SDL library is safe to use on older Macs

Also generate iOS versions of the Metal shaders

configure.in

@@ -2056,8 +2056,8 @@ AC_HELP_STRING([--enable-video-cocoa], [use Cocoa video driver [[default=yes]]])
 CheckMETAL()
 {
     AC_ARG_ENABLE(render-metal,
-AC_HELP_STRING([--enable-render-metal], [enable the Metal render driver [[default=no]]]),
-                                , enable_render_metal=no)
+AC_HELP_STRING([--enable-render-metal], [enable the Metal render driver [[default=yes]]]),
+                                , enable_render_metal=yes)
     if test x$enable_render = xyes -a x$enable_render_metal = xyes; then
         dnl This should maybe make sure you have a supported SDK version.
         AC_DEFINE(SDL_VIDEO_RENDER_METAL, 1, [ ])
@@ -3794,6 +3794,10 @@ AC_HELP_STRING([--enable-render-d3d], [enable the Direct3D render driver [[defau
         EXTRA_LDFLAGS="$EXTRA_LDFLAGS -Wl,-framework,Cocoa"
         EXTRA_LDFLAGS="$EXTRA_LDFLAGS -Wl,-framework,Carbon"
         EXTRA_LDFLAGS="$EXTRA_LDFLAGS -Wl,-framework,IOKit"
+
+        if test x$enable_render = xyes -a x$enable_render_metal = xyes; then
+            EXTRA_LDFLAGS="$EXTRA_LDFLAGS -Wl,-weak_framework,QuartzCore -Wl,-weak_framework,Metal"
+        fi
         ;;
     *-nacl|*-pnacl)
         ARCH=nacl