configure: improve the help text for external libraries

Add a more accurate description of what the switches actually do (i.e.
allow using the given library, not enabling the corresponding
codecs etc.).

Replace the library descriptions, in many cases boilerplate text without
useful information, with a short summary of what the library does.

configure

@@ -185,54 +185,66 @@ Individual component options:
   --disable-filters        disable all filters
 
 External library support:
-  --enable-avisynth        enable reading of AviSynth script files [no]
+
-  --enable-bzlib           enable bzlib [autodetect]
+  Using any of the following switches will allow Libav to link to the
-  --enable-frei0r          enable frei0r video filtering
+  corresponding external library. All the components depending on that library
-  --enable-gnutls          enable gnutls [no]
+  will become enabled, if all their other dependencies are met and they are not
-  --enable-libbs2b         enable bs2b DSP library [no]
+  explicitly disabled. E.g. --enable-libwavpack will enable linking to
-  --enable-libcdio         enable audio CD grabbing with libcdio
+  libwavpack and allow the libwavpack encoder to be built, unless it is
-  --enable-libdc1394       enable IIDC-1394 grabbing using libdc1394
+  specifically disabled with --disable-encoder=libwavpack.
-                           and libraw1394 [no]
+
-  --enable-libdcadec       enable DCA decoding via libdcadec [no]
+  Note that only the system libraries are auto-detected. All the other external
-  --enable-libfaac         enable AAC encoding via libfaac [no]
+  libraries must be explicitly enabled.
-  --enable-libfdk-aac      enable AAC de/encoding via libfdk-aac [no]
+
-  --enable-libfontconfig   enable libfontconfig [no]
+  Also note that the following help text describes the purpose of the libraries
-  --enable-libfreetype     enable libfreetype [no]
+  themselves, not all their features will necessarily be usable by Libav.
-  --enable-libgsm          enable GSM de/encoding via libgsm [no]
+
-  --enable-libilbc         enable iLBC de/encoding via libilbc [no]
+  --enable-avisynth          video frameserver
-  --enable-libkvazaar      enable HEVC encoding via libkvazaar [no]
+  --enable-bzlib             bzip2 compression [autodetect]
-  --enable-libmp3lame      enable MP3 encoding via libmp3lame [no]
+  --enable-frei0r            video filtering plugins
-  --enable-libnpp          enable NVIDIA Performance Primitives-based code [no]
+  --enable-gnutls            crypto
-  --enable-libopencore-amrnb enable AMR-NB de/encoding via libopencore-amrnb [no]
+  --enable-libbs2b           Bauer stereophonic-to-binaural DSP
-  --enable-libopencore-amrwb enable AMR-WB decoding via libopencore-amrwb [no]
+  --enable-libcdio           audio CD input
-  --enable-libopencv       enable video filtering via libopencv [no]
+  --enable-libdc1394         IEEE 1394/Firewire camera input
-  --enable-libopenh264     enable H.264 encoding via OpenH264 [no]
+  --enable-libdcadec         DCA audio decoding
-  --enable-libopenjpeg     enable JPEG 2000 de/encoding via OpenJPEG [no]
+  --enable-libfaac           AAC audio encoding
-  --enable-libopus         enable Opus de/encoding via libopus [no]
+  --enable-libfdk-aac        AAC audio encoding/decoding
-  --enable-libpulse        enable Pulseaudio input via libpulse [no]
+  --enable-libfontconfig     font configuration and management
-  --enable-librtmp         enable RTMP[E] support via librtmp [no]
+  --enable-libfreetype       font rendering
-  --enable-libschroedinger enable Dirac de/encoding via libschroedinger [no]
+  --enable-libgsm            GSM audio encoding/decoding
-  --enable-libsnappy       enable Snappy compression [no]
+  --enable-libilbc           ILBC audio encoding/decoding
-  --enable-libspeex        enable Speex de/encoding via libspeex [no]
+  --enable-libkvazaar        HEVC video encoding
-  --enable-libtheora       enable Theora encoding via libtheora [no]
+  --enable-libmp3lame        MP3 audio encoding
-  --enable-libtwolame      enable MP2 encoding via libtwolame [no]
+  --enable-libnpp            CUDA processing
-  --enable-libvo-aacenc    enable AAC encoding via libvo-aacenc [no]
+  --enable-libopencore-amrnb AMR-NB audio encoding/decoding
-  --enable-libvo-amrwbenc  enable AMR-WB encoding via libvo-amrwbenc [no]
+  --enable-libopencore-amrwb AMR-WB audio decoding
-  --enable-libvorbis       enable Vorbis encoding via libvorbis [no]
+  --enable-libopencv         computer vision
-  --enable-libvpx          enable VP8 and VP9 de/encoding via libvpx [no]
+  --enable-libopenh264       H.264 video encoding/decoding
-  --enable-libwavpack      enable wavpack encoding via libwavpack [no]
+  --enable-libopenjpeg       JPEG 2000 image encoding/decoding
-  --enable-libwebp         enable WebP encoding via libwebp [no]
+  --enable-libopus           Opus audio encoding/decoding
-  --enable-libx264         enable H.264 encoding via x264 [no]
+  --enable-libpulse          Pulseaudio sound server
-  --enable-libx265         enable HEVC encoding via x265 [no]
+  --enable-librtmp           RTMP streaming
-  --enable-libxavs         enable AVS encoding via xavs [no]
+  --enable-libschroedinger   Dirac video encoding/decoding
-  --enable-libxcb          enable X11 grabbing using XCB [no]
+  --enable-libsnappy         snappy compression
-  --enable-libxcb-shm      enable X11 grabbing shm communication [auto]
+  --enable-libspeex          Speex audio encoding/decoding
-  --enable-libxcb-xfixes   enable X11 grabbing mouse rendering [auto]
+  --enable-libtheora         Theora video encoding/decoding
-  --enable-libxvid         enable Xvid encoding via xvidcore,
+  --enable-libtwolame        MP2 audio encoding
-                           native MPEG-4/Xvid encoder exists [no]
+  --enable-libvo-aacenc      AAC audio encoding
-  --enable-openssl         enable openssl [no]
+  --enable-libvo-amrwbenc    AMR-WB audio encoding
-  --enable-x11grab         enable X11 grabbing (legacy) [no]
+  --enable-libvorbis         Vorbis audio encoding/decoding
-  --enable-zlib            enable zlib [autodetect]
+  --enable-libvpx            VP* video encoding/decoding
+  --enable-libwavpack        Wavpack audio encoding/decoding
+  --enable-libwebp           WebP image encoding/decoding
+  --enable-libx264           H.264 video encoding
+  --enable-libx265           HEVC video encoding
+  --enable-libxavs           Chinese AVS video encoding
+  --enable-libxcb            X window system protocol communication
+  --enable-libxcb-shm        X11 shm communication [auto]
+  --enable-libxcb-xfixes     X11 mouse rendering [auto]
+  --enable-libxvid           MPEG-4 ASP video encoding
+  --enable-openssl           crypto
+  --enable-x11grab           X11 grabbing through xlib (legacy, use xcb instead)
+  --enable-zlib              compression [autodetect]
 
 Toolchain options:
   --arch=ARCH              select architecture [$arch]