build-sys: add --with-audio=auto
build-sys: add --with-audio=auto

Defaults to GStreamer on Windows, and PulseAudio otherwise atm.

flexVDI/spice-gtk,SPICE/spice-gtk,fgouget/spice-gtk,flexVDI/spice-gtk,elmarco/spice-gtk,guodong/spice-gtk,guodong/spice-gtk,elmarco/spice-gtk,freedesktop-unofficial-mirror/spice__spice-gtk,fgouget/spice-gtk,elmarco/spice-gtk,guodong/spice-gtk,SPICE/spice-gtk,guodong/spice-gtk,Fantu/spice-gtk,SPICE/spice-gtk,freedesktop-unofficial-mirror/spice__spice-gtk,elmarco/spice-gtk,fgouget/spice-gtk,Fantu/spice-gtk,SPICE/spice-gtk,Fantu/spice-gtk,freedesktop-unofficial-mirror/spice__spice-gtk,Fantu/spice-gtk,freedesktop-unofficial-mirror/spice__spice-gtk