vtenc: VTCompressionSessionPrepareToEncodeFrames only exists since 10.9.6

Check with configure for it instead of using one of the availability macros
as those wouldn't work as expected with minor versions.

configure.ac

@@ -504,6 +504,16 @@ AC_CHECK_HEADER(AVFoundation/AVFoundation.h, HAVE_AVFOUNDATION="yes", HAVE_AVFOU
 AC_CHECK_HEADER(MobileCoreServices/MobileCoreServices.h, HAVE_IOS="yes", HAVE_IOS="no", [-])
 AC_CHECK_HEADER(VideoToolbox/VideoToolbox.h, HAVE_VIDEOTOOLBOX="yes", HAVE_VIDEOTOOLBOX="no", [-])
 
+if test "x$HAVE_VIDEOTOOLBOX" = "xyes"; then
+  old_LIBS=$LIBS
+  LIBS="$LIBS -framework VideoToolbox"
+  AC_CHECK_FUNC(VTCompressionSessionPrepareToEncodeFrames, [
+    AC_DEFINE(HAVE_VIDEOTOOLBOX_10_9_6, 1, [Define if building with VideoToolbox >= 10.9.6])
+  ], [
+  ])
+  LIBS=$old_LIBS
+fi
+
 AM_CONDITIONAL(HAVE_AVFOUNDATION, test "x$HAVE_AVFOUNDATION" = "xyes")
 if test "x$HAVE_AVFOUNDATION" = "xyes"; then
   AC_DEFINE(HAVE_AVFOUNDATION, 1, [Define if building with AVFoundation])

sys/applemedia/vtenc.c

@@ -492,12 +492,16 @@ gst_vtenc_create_session (GstVTEnc * self)
   gst_vtenc_session_configure_bitrate (self, session,
       gst_vtenc_get_bitrate (self));
 
-  status = VTCompressionSessionPrepareToEncodeFrames (session);
-  if (status != noErr) {
-    GST_ERROR_OBJECT (self,
-        "VTCompressionSessionPrepareToEncodeFrames() returned: %d",
-        (int) status);
+#ifdef HAVE_VIDEOTOOLBOX_10_9_6
+  if (VTCompressionSessionPrepareToEncodeFrames) {
+    status = VTCompressionSessionPrepareToEncodeFrames (session);
+    if (status != noErr) {
+      GST_ERROR_OBJECT (self,
+          "VTCompressionSessionPrepareToEncodeFrames() returned: %d",
+          (int) status);
+    }
   }
+#endif
 
 beach:
   CFRelease (pb_attrs);