Add the audio handling changes to build system and documentation

INSTALL

@@ -1,4 +1,4 @@
-Things needed for compilation.
+DOSBox uses the following libraries:
 
 SDL 
     The Simple DirectMedia Library available at http://www.libsdl.org
@@ -10,6 +10,25 @@ SDL
     Note that only version 1.2 and its subversions (1.2.8, 1.2.13 etc.)
     are currently supported.
 
+Opusfile, by Xiph
+    For compressed audio tracks (.opus) used with CDROM images.
+    Sources are available at https://opus-codec.org, however it is
+    also conveniently packages by all popular package managers for
+    Windows (MSYS2, MinGW, and Chocolatey), Linux (apt, dnf, zypper,
+    pacman), and OS X (Homebrew and MacPorts).  Opus is today's
+    leading compression format and has replaced Vorbis as Ogg's
+    recommended lossy format. Is widely used in the largest audio and
+    video distribution platforms such as YouTube.
+    License: three-clause BSD
+
+SpeexDSP, by Xiph
+    Needed to perform on-the-fly resampling of Opus-compressed CDROM
+    audio data in the event DOSBox's mixer sampling rate differs from
+    that of the Opus sampling rate. Sources are available at
+    https://opus-codec.org, however it is also conveniently packages
+    by all popular package managers.
+    License: three-clause BSD
+
 Curses (optional)
     If you want to enable the debugger you need a curses library.
     ncurses should be installed on just about every unix distro.
@@ -32,19 +51,18 @@ SDL_Net (optional)
     For modem/ipx support. Get it from http://www.libsdl.org/projects/SDL_net/
     Licensed under LGPL
 
-SDL_Sound 
-    For compressed audio on diskimages. (optional)
-    This is for cue/bin cdrom images with compressed (mp3/ogg) audio tracks.
-    Get it from http://icculus.org/SDL_sound
-    Licenced under LGPL
-
-ALSA_Headers
-    (optional)
-    for Alsa support under linux. Part of the linux kernel sources
+alsa-lib (optional)
+    For ALSA audio support under linux. Get it from https://www.alsa-project.org/
     Licensed under LGPL
 
-If you want compile from developer sources (SVN) under a unix system, you'll also need 
-automake (>=1.6), autoconf(>=2.50). Should be available at http://www.gnu.org
+If you want compile from developer sources (SVN) under a unix system, you will need:
+ - Subversion to checkout the sources, or gzip and tar to unpack them from archive
+ - GCC (>=4.8.1) or Clang (>=3.3)
+ - automake (>=1.6)
+ - autoconf (>=2.50)
+ - autoconf-archive (>=2009.x)
+ - make (>= 3.8)
+ - pkg-config (>= 0.25)
 
 For building on unix systems.
 If you are building from developer sources run ./autogen.sh first before doing the following.