pjsip on MacOS doesn't use CoreAudio

[samuelv@xxxxxxxxxxx (Samuel Vinson)]

Hello,

Currently, on MacOS aconfigure.ac define portaudio as audio back end and 
not CoreAudio.
You should change the following line:

   *darwin*)
       ...
       ac_pjmedia_snd=pa_darwinos
       ...

by

   *darwin*)
       ...
       ac_pjmedia_snd=null
       ...



Best regards

Samuel