build: do not build virtio-vga-gl if virgl/opengl not available

If virgl and opengl are not available, the build process creates a useless
libvirtio-vga-gl module that does not have any device in it.  Follow the
example of virtio-vga-rutabaga and do not build the module at all in that
case.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
1 file changed