make it possible to compile ffmpeg without swscale

Signed-off-by: Paul B Mahol <onemda@gmail.com>

configure

@@ -2263,7 +2263,7 @@ postproc_deps="avutil gpl"
 swscale_deps="avutil"
 
 # programs
-ffmpeg_deps="avcodec avfilter avformat swscale swresample"
+ffmpeg_deps="avcodec avfilter avformat swresample"
 ffmpeg_select="aformat_filter anull_filter atrim_filter format_filter
                null_filter
                setpts_filter trim_filter"

ffmpeg_opt.c

@@ -2457,7 +2457,9 @@ void show_help_default(const char *opt, const char *arg)
         int flags = AV_OPT_FLAG_DECODING_PARAM | AV_OPT_FLAG_ENCODING_PARAM;
         show_help_children(avcodec_get_class(), flags);
         show_help_children(avformat_get_class(), flags);
+#if CONFIG_SWSCALE
         show_help_children(sws_get_class(), flags);
+#endif
         show_help_children(swr_get_class(), AV_OPT_FLAG_AUDIO_PARAM);
         show_help_children(avfilter_get_class(), AV_OPT_FLAG_VIDEO_PARAM | AV_OPT_FLAG_AUDIO_PARAM | AV_OPT_FLAG_FILTERING_PARAM);
     }