shairport-sync: needs toolchain with NPTL

Also remove the comment attributing threads dependency to alsa-lib, since
shairport-sync itself uses pthreads.

Fixes:
http://autobuild.buildroot.net/results/58e/58eafd2499919a45bfb5a2becc233d3678deedde/



Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>