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