data/dconf: Don't run "dconf update" if $(DESTDIR) is set

dconf changed as of
https://git.gnome.org/browse/dconf/commit/?id=c211fc46496597c7ddabd73d623bae4037754916
to actually emit an error if /etc/dconf/db is empty.  When building
ibus in a system such as dpkg/rpm or gnome-continuous, there may
actually be nothing in that directory in the buildroot.

This will now cause "dconf update" as executed by this Makefile to
fail.  The fix is to just check $(DESTDIR), like we should do for all
triggers (e.g. gtk-update-icon-cache too).  It's never useful to
execute these from per-component Makefiles if $(DESTDIR) is set.

Instead, these meta-build systems (dpkg/rpm/jhbuild/Continuous) all
take care of execution of triggers on their own.

Review URL: https://codereview.appspot.com/51730044
Patch from Colin Walters <walters@verbum.org>.