Until now we'd get pointless messages like:
warning: collision encountered:
/gnu/store/…-gtk-icon-themes/share/icons/hicolor/icon-theme.cache
/gnu/store/…-inkscape-0.92.3/share/icons/hicolor/icon-theme.cache
warning: choosing /gnu/store/…-gtk-icon-themes/share/icons/hicolor/icon-theme.cache
* guix/build/union.scm (%harmless-collisions): New variable.
(warn-about-collision): Honor it.