madplay: fix alsa detection/linking using host-pkgconf

Fixes [1]: configure check for libasound failed because of missing
pthread linkage.

[1] http://autobuild.buildroot.net/results/f3a/f3a0b3ff15e729d223c8c8ee6af75c425cd344b6/



Signed-off-by: Peter Seiderer <ps.report@gmx.net>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>