portaudio: fix static linking issue with portaudio

Fixes:
http://autobuild.buildroot.net/results/25f/25f8088ecb276ff4559a56fcc4f1b92d11f7deac/



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

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>