Improve configure help text.

Show which features external libraries enable.
Fixes ticket #4157

configure

@@ -194,25 +194,27 @@ Individual component options:
 External library support:
   --enable-avisynth        enable reading of AviSynth script files [no]
   --disable-bzlib          disable bzlib [autodetect]
-  --enable-fontconfig      enable fontconfig
-  --enable-frei0r          enable frei0r video filtering
-  --enable-gnutls          enable gnutls [no]
+  --enable-fontconfig      enable fontconfig, useful for drawtext filter [no]
+  --enable-frei0r          enable frei0r video filtering [no]
+  --enable-gnutls          enable gnutls, needed for https support
+                           if openssl is not used [no]
   --disable-iconv          disable iconv [autodetect]
-  --enable-ladspa          enable LADSPA audio filtering
+  --enable-ladspa          enable LADSPA audio filtering [no]
   --enable-libaacplus      enable AAC+ encoding via libaacplus [no]
-  --enable-libass          enable libass subtitles rendering [no]
+  --enable-libass          enable libass subtitles rendering,
+                           needed for subtitles and ass filter [no]
   --enable-libbluray       enable BluRay reading using libbluray [no]
   --enable-libbs2b         enable bs2b DSP library [no]
-  --enable-libcaca         enable textual display using libcaca
+  --enable-libcaca         enable textual display using libcaca [no]
   --enable-libcelt         enable CELT decoding via libcelt [no]
-  --enable-libcdio         enable audio CD grabbing with libcdio
+  --enable-libcdio         enable audio CD grabbing with libcdio [no]
   --enable-libdc1394       enable IIDC-1394 grabbing using libdc1394
                            and libraw1394 [no]
   --enable-libfaac         enable AAC encoding via libfaac [no]
   --enable-libfdk-aac      enable AAC de/encoding via libfdk-aac [no]
   --enable-libflite        enable flite (voice synthesis) support via libflite [no]
-  --enable-libfreetype     enable libfreetype [no]
-  --enable-libfribidi      enable libfribidi [no]
+  --enable-libfreetype     enable libfreetype, needed for drawtext filter [no]
+  --enable-libfribidi      enable libfribidi, improves drawtext filter [no]
   --enable-libgme          enable Game Music Emu via libgme [no]
   --enable-libgsm          enable GSM de/encoding via libgsm [no]
   --enable-libiec61883     enable iec61883 via libiec61883 [no]
@@ -264,7 +266,8 @@ External library support:
   --enable-openal          enable OpenAL 1.1 capture support [no]
   --enable-opencl          enable OpenCL code
   --enable-opengl          enable OpenGL rendering [no]
-  --enable-openssl         enable openssl [no]
+  --enable-openssl         enable openssl, needed for https support
+                           if gnutls is not used [no]
   --disable-sdl            disable sdl [autodetect]
   --enable-x11grab         enable X11 grabbing (legacy) [no]
   --disable-xlib           disable xlib [autodetect]