Build: do not check for gnome-icon-theme presence.

This is not actually a build dependency, but was introduced to 'ensure
users have it available when they build to avoid bugs on execution'

With GNOME 3.14 though, gnome-icon-theme no longer exists (merged and
renamed to adwaita-icon-theme).

https://bugzilla.gnome.org/show_bug.cgi?id=735275
common: Change icon loading behaviour

Signed-off-by: Jens Georg <mail@jensge.org>

https://bugzilla.gnome.org/show_bug.cgi?id=735275
av-cp: Fix warnings about transient dialog

Signed-off-by: Jens Georg <mail@jensge.org>