configure: Sort the library listings in the help text alphabetically

Only these three libraries were out of order, the rest was already
neatly sorted.
Signed-off-by: Martin Storsjö <martin@martin.st>

configure

@@ -166,9 +166,6 @@ External library support:
   --enable-bzlib           enable bzlib [autodetect]
   --enable-frei0r          enable frei0r video filtering
   --enable-gnutls          enable gnutls [no]
-  --enable-libopencore-amrnb enable AMR-NB de/encoding via libopencore-amrnb [no]
-  --enable-libopencore-amrwb enable AMR-WB decoding via libopencore-amrwb [no]
-  --enable-libopencv       enable video filtering via libopencv [no]
   --enable-libcdio         enable audio CD grabbing with libcdio
   --enable-libdc1394       enable IIDC-1394 grabbing using libdc1394
                            and libraw1394 [no]
@@ -176,6 +173,9 @@ External library support:
   --enable-libfreetype     enable libfreetype [no]
   --enable-libgsm          enable GSM support via libgsm [no]
   --enable-libmp3lame      enable MP3 encoding via libmp3lame [no]
+  --enable-libopencore-amrnb enable AMR-NB de/encoding via libopencore-amrnb [no]
+  --enable-libopencore-amrwb enable AMR-WB decoding via libopencore-amrwb [no]
+  --enable-libopencv       enable video filtering via libopencv [no]
   --enable-libopenjpeg     enable JPEG 2000 decoding via OpenJPEG [no]
   --enable-libpulse        enable Pulseaudio input via libpulse [no]
   --enable-librtmp         enable RTMP[E] support via librtmp [no]