Log Message:
ffmpeg4: add pulseaudio option to enable libpulse support.

With this option and xcb option (for x11grab) I can capture X11 applications
that use pulseaudio by the following ffmpeg4 command options:
% ffmpeg4 -y -f x11grab -video_size 800x600 -r 24 -i :0.0+0,0 \
  -f pulse -i oss_output.audio.monitor -pix_fmt yuv420p capture.mp4