Fix small errors in configure --help's output.

Originally committed as revision 16429 to svn://svn.ffmpeg.org/ffmpeg/trunk

configure

@@ -91,9 +91,9 @@ show_help(){
   echo "External library support:"
   echo "  --enable-mlib            enable Sun medialib [no]"
   echo "  --enable-avisynth        enable reading of AVISynth script files [no]"
-  echo "  --enable-bzlib           enable bzlib [no]"
-  echo "  --enable-libamr-nb       enable libamr-nb floating point audio codec"
-  echo "  --enable-libamr-wb       enable libamr-wb floating point audio codec"
+  echo "  --enable-bzlib           enable bzlib [autodetect]"
+  echo "  --enable-libamr-nb       enable libamr-nb floating point audio codec [no]"
+  echo "  --enable-libamr-wb       enable libamr-wb floating point audio codec [no]"
   echo "  --enable-libdc1394       enable IIDC-1394 grabbing using libdc1394"
   echo "                           and libraw1394 [no]"
   echo "  --enable-libdirac        enable Dirac support via libdirac [no]"
@@ -112,7 +112,7 @@ show_help(){
   echo "  --enable-libx264         enable H.264 encoding via x264 [no]"
   echo "  --enable-libxvid         enable Xvid encoding via xvidcore,"
   echo "                           native MPEG-4/Xvid encoder exists [no]"
-  echo "  --enable-zlib            enable zlib [no]"
+  echo "  --enable-zlib            enable zlib [autodetect]"
   echo ""
   echo "Advanced options (experts only):"
   echo "  --source-path=PATH       path to source code [$source_path]"