Clarify usage of external libs whose functionality exists natively.

Originally committed as revision 7771 to svn://svn.ffmpeg.org/ffmpeg/trunk

configure

@@ -87,12 +87,15 @@ show_help(){
   echo "  --enable-faadbin         build FAAD support with runtime linking [default=no]"
   echo "  --enable-libgsm          enable GSM support via libgsm [default=no]"
   echo "  --enable-mp3lame         enable MP3 encoding via libmp3lame [default=no]"
-  echo "  --enable-libnut          enable NUT support via libnut [default=no]"
+  echo "  --enable-libnut          enable NUT (de)muxing via libnut, native demuxer"
+  echo "                           exists [default=no]"
   echo "  --enable-libogg          enable Ogg muxing via libogg [default=no]"
   echo "  --enable-libtheora       enable Theora encoding via libtheora [default=no]"
-  echo "  --enable-vorbis          enable Vorbis en/decoding via libvorbis [default=no]"
+  echo "  --enable-vorbis          enable Vorbis en/decoding via libvorbis, native"
+  echo "                           implementations exist [default=no]"
   echo "  --enable-x264            enable H.264 encoding via x264 [default=no]"
-  echo "  --enable-xvid            enable Xvid encoding via xvidcore [default=no]"
+  echo "  --enable-xvid            enable Xvid encoding via xvidcore, native"
+  echo "                           MPEG-4/Xvid encoder exists [default=no]"
   echo "  --enable-amr_nb          enable amr_nb float audio codec"
   echo "  --enable-amr_nb-fixed    use fixed point for amr-nb codec"
   echo "  --enable-amr_wb          enable amr_wb float audio codec"