build: add configure option to control vfs_snapper build

build: add configure option to control vfs_snapper build

vfs_snapper is currently built if dbus development headers / libraries
are detected during configure. This commit adds new --disable-snapper
and --enable-snapper (default) configure parameters. When enabled,
configure will fail if the dbus development headers / libraries are
missing.

Signed-off-by: Matt Taylor <liverbugg@rinux.org>
Reviewed-by: David Disseldorp <ddiss@samba.org>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
Autobuild-User(master): Andrew Bartlett <abartlet@samba.org>
Autobuild-Date(master): Mon May 25 01:16:46 UTC 2020 on sn-devel-184