Update dconf to 0.40.0

XXX Please check on illumos:

The patches for illumos as per

https://gitlab.gnome.org/GNOME/dconf/issues/49

no longer apply. It is not obvious to me whether or not they are still
necessary given the new use of symbol files.
(Patch author contacted a week ago.)

Changes in dconf 0.40.0
=======================

- common: Add missing G_BEGIN/END_DECLS to allow use of headers from C++
  code (Philip Withnall, !67)

Changes in dconf 0.39.1
=======================

- build: improve libdconf visible symbols (Inigo Martinez, Daniel Playfair
  Cal, !59)
- engine: do not emit optimistic change notifications unless the local value
  is different (Daniel Playfair Cal, !2)
- build: minor fixes to bash completion script (Philip Withnall, !64)
- service: add a systemd unit for D-Bus activation (Simon McVittie, !63)

Changes in dconf 0.38.0
=======================

- build: Install bash-completion relative to datadir (Jan Trojnar, !58)
- client: add `dconf compile` to shell autocomplete (Andreas Polnas, !60)

(prlw1)