Add libmad package for MP3 support.

At request of Fabian.

pkgdef/SDL2_mixer.sh

@@ -9,8 +9,8 @@ variant latest fetch_hg https://hg.libsdl.org/SDL_mixer
 
 # Disable dependencies on external libraries for sound file formats:
 config_options="
-    --disable-music-mod --disable-music-mp3
-    --disable-music-flac-shared --disable-music-ogg-shared
+    --disable-music-mod --disable-music-flac-shared
+    --disable-music-ogg-shared
 "
 
 # ...except ones we have installed:
@@ -28,4 +28,10 @@ else
     config_options+=" --disable-music-midi-fluidsynth"
 fi
 
+if chocpkg installed ${PACKAGE_TYPE}:libmad; then
+    config_options+=" --enable-music-mp3 --enable-music-mp3-mad-gpl"
+else
+    config_options+=" --disable-music-mp3 --disable-music-mp3-mad-gpl"
+fi
+
 build_autotools $config_options

pkgdef/libmad.sh

@@ -0,0 +1,25 @@
+description "MPEG Audio Decoder library"
+check_library mad
+variant stable fetch_download \
+    ftp://ftp.mars.org/pub/mpeg/libmad-0.15.1b.tar.gz \
+    bbfac3ed6bfbc2823d3775ebb931087371e142bb0e9bb1bee51a76a6e0078690
+build_autotools
+
+prebuild_setup() {
+  cp configure configure.orig
+  # libmad hasn't had a new release in 14 years, so we must manually
+  # patch the configure script to remove all these compiler flags which
+  # don't work any more (especially in clang):
+  sed <configure.orig >configure '
+    s/-fforce-mem//
+    s/-fthread-jumps//
+    s/-fcse-follow-jumps//
+    s/-fcse-skip-blocks//
+    s/-fregmove//
+    s/-fexpensive-optimizations//
+    s/-fschedule-insns2//
+    s/-fstrength-reduce//
+    s/-march=i486//
+  '
+}
+