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