build-sys: disable celt051 by default when configure from autogen
build-sys: disable celt051 by default when configure from autogen

Avoid the new error message for explicit celt051 option.

Signed-off-by: Marc-André Lureau <a6f2b960a4aca61381fc650a67901b2ac8e83e6d@redhat.com>

flexVDI/spice-gtk,flexVDI/spice-gtk