[patch] external MPEG audio decoding using libmpg123

Hi all,

this is an updated version of my patch to enable use of libmpg123 from
lame, facilitating future deprecation of the internal mpglib fork. With
the patch, a suitable libmpg123 is detected and replaces mpglib by
default. Configure switches override that.

The necessary changes to the libmpg123 API are in mpg123 development
trunk and I am waiting with a release until it is confirmed that the
API additions are good for LAME use (namely, mp3x). So you need a
procedure along this to test the patch:

#!/bin/sh
set -e
workdir=$(mktemp -d -t lametest.XXXXX)
echo "working directory: $workdir"
cd $workdir
# creates mpg123-$TIMESTAMP.tar.bz2
# Alternatively, fetch svn://scm.orgis.org/mpg123/trunk and
# run `autoreconf -iv` there.
wget --content-disposition https://mpg123.org/snapshot
tar -xf mpg123-*.tar.bz2
cd mpg123-*/
./configure --prefix=$workdir/mpg123
make -j3
make install
cd $workdir
wget https://mpg123.org/test/lame-svn-r6432-libmpg123.patch
svn co https://svn.code.sf.net/p/lame/svn/trunk/lame lame
cd lame
patch -Np0 < ../lame-svn-r6432-libmpg123.patch
autoreconf
# gnu89 needed for mp3x, with gtk1
# If you do not care, remove --enable-mp3x.
CFLAGS="-std=gnu89 $CFLAGS" \
PKG_CONFIG_PATH=$workdir/mpg123/lib/pkgconfig:$PKG_CONFIG_PATH \
./configure --enable-mp3x --enable-libmpg123
make -j3
pwd
# Now test things in frontend/.

Once it is confirmed that we can fix the libmpg123 API, I will release
version 1.26.0 and a subsequent LAME release can rely on this official
mpg123 version.

I manually specified the list of files to svn diff to avoid including
all the autotool noise.

Please review the patch for all the bad hackery that I have committed
and test if it works despite all ugliness;-)


Alrighty then,

Thomas
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