build: disable PortAudio by default

This was originally added because we thought this would make a good
portable audio API, which would give us good behavior on Windows, Linux,
and OSX. But this hope was disappointed: it's not reliable enough (nice
deadlocks on Linux when seeking, i.e. resetting the audio device),
doesn't have enough features (no channel maps, no digital passthrough),
and in general just is not very good.

wscript

@@ -473,6 +473,7 @@ audio_output_features = [
         'desc': 'PortAudio audio output',
         'deps': [ 'pthreads' ],
         'func': check_pkg_config('portaudio-2.0', '>= 19'),
+        'default': 'disable',
     }, {
         'name': '--jack',
         'desc': 'JACK audio output',