Avoid using undefined symbols.

This fixes the following option set:
  --disable-encoders --disable-decoders --disable-parsers --disable-bsfs
  --disable-muxers --disable-demuxers --disable-protocols
  --enable-muxer=mpeg1system

Originally committed as revision 9650 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavformat/mpegenc.c

@@ -293,10 +293,12 @@ static int mpeg_mux_init(AVFormatContext *ctx)
     int video_bitrate;
 
     s->packet_number = 0;
-    s->is_vcd = (ctx->oformat == &mpeg1vcd_muxer);
-    s->is_svcd = (ctx->oformat == &mpeg2svcd_muxer);
-    s->is_mpeg2 = (ctx->oformat == &mpeg2vob_muxer || ctx->oformat == &mpeg2svcd_muxer || ctx->oformat == &mpeg2dvd_muxer);
-    s->is_dvd = (ctx->oformat == &mpeg2dvd_muxer);
+    s->is_vcd =   (ENABLE_MPEG1VCD_MUXER  && ctx->oformat == &mpeg1vcd_muxer);
+    s->is_svcd =  (ENABLE_MPEG2SVCD_MUXER && ctx->oformat == &mpeg2svcd_muxer);
+    s->is_mpeg2 = (ENABLE_MPEG2VOB_MUXER  && ctx->oformat == &mpeg2vob_muxer ||
+                   ENABLE_MPEG2DVD_MUXER  && ctx->oformat == &mpeg2dvd_muxer ||
+                   ENABLE_MPEG2SVCD_MUXER && ctx->oformat == &mpeg2svcd_muxer);
+    s->is_dvd =   (ENABLE_MPEG2DVD_MUXER  && ctx->oformat == &mpeg2dvd_muxer);
 
     if(ctx->packet_size)
         s->packet_size = ctx->packet_size;