build: toggle vfs_snapper using --with-shared-modules

build: toggle vfs_snapper using --with-shared-modules

7ae03a19b3c ("build: add configure option to control vfs_snapper build")
added new --enable-snapper and --disable-snapper configure parameters to
control whether the vfs_snapper module was built.
The new parameters conflicted with existing
--with-shared-modules=[!]vfs_snapper behaviour.

This change reinstates working --with-shared-modules=[!]vfs_snapper
functionality. vfs_snapper stays enabled by default, but only on Linux.
Linux systems lacking the dbus library and header files should
explicitly disable the module via --with-shared-modules=!vfs_snapper as
documented.

Bug: https://bugzilla.samba.org/show_bug.cgi?id=14437

Signed-off-by: David Disseldorp <ddiss@samba.org>
Reviewed-by: Björn Jacke <bjacke@samba.org>
Autobuild-User(master): David Disseldorp <ddiss@samba.org>
Autobuild-Date(master): Wed Sep  2 16:24:50 UTC 2020 on sn-devel-184

(cherry picked from commit b6805d5e0bcf1716f87e84bcbb2fd8f93c38a8a3)

Autobuild-User(v4-13-test): Karolin Seeger <kseeger@samba.org>
Autobuild-Date(v4-13-test): Fri Sep  4 12:37:35 UTC 2020 on sn-devel-184