package/mplayer: add optional support for mpg123

package/mplayer: add optional support for mpg123

mplayer has optional support for mpg123, to get reproducable builds
add mpg123 as optional dependency.

Linked libraries without this patch:
$ output/host/usr/bin/x86_64-linux-readelf -a output/target/usr/bin/mplayer | grep NEEDED
 0x0000000000000001 (NEEDED)             Shared library: [librt.so.1]
 0x0000000000000001 (NEEDED)             Shared library: [libpthread.so.1]
 0x0000000000000001 (NEEDED)             Shared library: [libdl.so.1]
 0x0000000000000001 (NEEDED)             Shared library: [libm.so.1]
 0x0000000000000001 (NEEDED)             Shared library: [libc.so.1]

Linked libraries after this patch:
$ output/host/usr/bin/x86_64-linux-readelf -a output/target/usr/bin/mplayer | grep NEEDED
 0x0000000000000001 (NEEDED)             Shared library: [librt.so.1]
 0x0000000000000001 (NEEDED)             Shared library: [libmpg123.so.0]
 0x0000000000000001 (NEEDED)             Shared library: [libpthread.so.1]
 0x0000000000000001 (NEEDED)             Shared library: [libdl.so.1]
 0x0000000000000001 (NEEDED)             Shared library: [libm.so.1]
 0x0000000000000001 (NEEDED)             Shared library: [libc.so.1]

Like with many other optional libraries detection of mpg123 is broken if
--enable-mpg123 is passed to configure leading to a build error:

libmpcodecs/ad_mpg123.o: In function `set_format':
ad_mpg123.c:(.text+0x63): undefined reference to `mpg123_getformat'
libmpcodecs/ad_mpg123.o: In function `decode_a_bit':
ad_mpg123.c:(.text+0x1b6): undefined reference to `mpg123_feed'
ad_mpg123.c:(.text+0x20e): undefined reference to `mpg123_replace_buffer'
ad_mpg123.c:(.text+0x223): undefined reference to `mpg123_decode_frame_64'
ad_mpg123.c:(.text+0x275): undefined reference to `mpg123_strerror'
[...]

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>