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>

package/mpg123/mpg123.mk

@@ -52,6 +52,9 @@ ifeq ($(BR2_PACKAGE_PORTAUDIO),y)
 MPG123_AUDIO += portaudio
 MPG123_CONF_OPTS += --with-default-audio=portaudio
 MPG123_DEPENDENCIES += portaudio
+# configure script does NOT use pkg-config to figure out how to link
+# with portaudio, breaking static linking as portaudio uses pthreads
+MPG123_CONF_ENV += LIBS='$(shell $(PKG_CONFIG_HOST_BINARY) --libs portaudio-2.0)'
 endif
 
 ifeq ($(BR2_PACKAGE_SDL),y)