build: add configure option to control vfs_snapper build

As discussed on the mailing list via https://lists.samba.org/archive/samba-technical/2020-May/135241.html

I've added one extra change to back out the vfs_snapper-enabled-by-default behaviour. I was a little too worried about fallout from automated build scripts and don't expect that vfs_snapper is that widely used.