Use triggers instead of enforcing the INSTALL_ICONS which enforces
gtk-update-icon-cache dependency on every ports installing icons.
The bonus:
- factorize gtk-update-icon-cache update at the end of the build
- stop adding a gratuicious dependency on gtk-update-icon-cache on packages which do not need it
- works on packages where INSTALLS_ICONS were forgotten
Since you're touching that line anyway, maybe s/finer granularity/shorter interval/ at the same time?