lavc/libx264: add a warning when using qscale

This addresses ticket #3238.
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/libx264.c

@@ -330,6 +330,9 @@ static av_cold int X264_init(AVCodecContext *avctx)
     X264Context *x4 = avctx->priv_data;
     int sw,sh;
 
+    if (avctx->global_quality > 0)
+        av_log(avctx, AV_LOG_WARNING, "-qscale is ignored, -crf is recommended.\n");
+
     x264_param_default(&x4->params);
 
     x4->params.b_deblocking_filter         = avctx->flags & CODEC_FLAG_LOOP_FILTER;