package/mpg123: fix static linking issue with alsa

package/mpg123: fix static linking issue with alsa

As opposed to the other audio backends, the configure script does NOT
use pkg-config to figure out how to link with alsa, breaking static
linking as alsa uses pthreads.

This patch uses the same fix from Peter used for portaudio:
http://git.buildroot.net/buildroot/commit/package/mpg123/mpg123.mk?id=dea306c78b5d4c571555c50512e212a725b594b9

Fixes
http://autobuild.buildroot.net/results/796/79613fda3f51e969ecd46ae1151bd4c3316639c3/
http://autobuild.buildroot.net/results/b0a/b0a99ff1516602ed42c0cc14d9d2922a01cdf88f/
http://autobuild.buildroot.net/results/5bd/5bdfd2aa9de568d0f7be27ffb18d4541b0e1be0c/
and many others

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