option: use more portable default ca-file path
option: use more portable default ca-file path

If no CA path is given, a default one is set. Use g_build_filename() to
set a more portable and valid default path.

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