Add SPICE_MAX_CLIPBOARD environment variable
Add SPICE_MAX_CLIPBOARD environment variable

Allow to easily override default max-clipboard value with environment
variable.

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