Allow Opus CDDA support to be optionally disabled

Adds a `--disable-opus-cdda` flag that explicitly disables support
for Ogg Opus CDDA tracks and in turn avoid the need for the Opus package
dependencies such as the opusfile, opus, and ogg libraries.

This feature does not alter the default operation of ./configure, which
is to enable Opus CDDA support and quit if the Opus dependency package,
opusfile, is not found.  The user can then choose to either a) install
the package or b) explicitly disable Opus support.

This commit also includes:

- fixes for a double-free in the MP3 close routine that
  was discovered during testing

- a message if a CD audio track cannot be added during CDROM
  mounting (such as attempting to use Opus tracks when the binary
  does not support them).

- the --disable-opus-cdda flag in our config heavy workflow

configure.ac

@@ -459,10 +459,23 @@ else
    AC_MSG_WARN([Can't find SDL_net, internal modem and ipx disabled])
 fi
 
-# Check for the Opus file-handling library
-PKG_CHECK_MODULES([OPUSFILE], [opusfile],
-  [ LIBS="$LIBS ${OPUSFILE_LIBS}"
-    CPPFLAGS="$CPPFLAGS ${OPUSFILE_CFLAGS}" ], [])
+dnl Ogg Opus handling
+dnl -----------------
+dnl    We want Opus tracks supported by default, and we also want the user
+dnl    to be aware if there's a problem finding the Opus dependencies.
+dnl    If there's a problem, we want the user to either a) install the opus
+dnl    library or b) explicitly disable Opus.
+
+dnl    To achieve this, we provide a --disable-opus-cdda configure option
+dnl    but by default we look for Opus and fail if we can't find it.
+dnl    We only skip supporting Opus if the user explicitly passes the
+dnl    --disable-opus-cdda argument.
+
+AC_ARG_ENABLE([opus-cdda], AS_HELP_STRING([--disable-opus-cdda],
+            [Disable support for Opus-compressed CD-Audio tracks]))
+AS_IF([test "x$enable_opus_cdda" != "xno"],
+      [PKG_CHECK_MODULES([OPUSFILE], [opusfile], [HAVE_OPUSFILE=1])])
+AM_CONDITIONAL([USE_OPUS], [test "$HAVE_OPUSFILE" == "1"])
 
 AH_TEMPLATE(C_OPENGL,[Define to 1 to use opengl display output support])
 AC_CHECK_LIB(GL, main, have_gl_lib=yes, have_gl_lib=no , )