Don't run gtk-update-icon-cache if DESTDIR is set

We don't want to run gtk-update-icon-cache during a package build, when
we're installing into a staging directory just to create a tree that will
be packaged up and really installed later.  The standard heuristic seems
to be to skip gtk-update-icon-cache if DESTDIR is set (ie we're installing
to a different location than we're configured to run from).

Signed-off-by: Roland Dreier <roland@digitalvampire.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>