alsa-lib: require thread support

Some parts of alsa-lib correctly use threads only when available, but
some other parts, especially certain PCM plugins, unconditionally
require threads. While it would certainly be possible to fix alsa-lib
to only use threads when available, it probably doesn't make much
sense, since on an embedded system that has audio, we are probably
powerful enough to enable thread support in the C library.

Fixes:

  http://autobuild.buildroot.org/results/e14469be7f6171f4c8c0c09c8e32943819f7938b/build-end.log



Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>