union: Do not warn about harmless collisions.

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.

guix/build/union.scm

@@ -81,14 +81,23 @@ identical, #f otherwise."
                                    (or (eof-object? n1)
                                        (loop))))))))))))))
 
+(define %harmless-collisions
+  ;; This is a list of files that are known to collide, but for which emitting
+  ;; a warning doesn't make sense.  For example, "icon-theme.cache" is
+  ;; regenerated by a profile hook which shadows the file provided by
+  ;; individual packages, and "gschemas.compiled" is made available to
+  ;; applications via 'glib-or-gtk-build-system'.
+  '("icon-theme.cache" "gschemas.compiled"))
+
 (define (warn-about-collision files)
   "Handle the collision among FILES by emitting a warning and choosing the
 first one of THEM."
-  (format (current-error-port)
-          "~%warning: collision encountered:~%~{  ~a~%~}"
-          files)
   (let ((file (first files)))
-    (format (current-error-port) "warning: choosing ~a~%" file)
+    (unless (member (basename file) %harmless-collisions)
+      (format (current-error-port)
+              "~%warning: collision encountered:~%~{  ~a~%~}"
+              files)
+      (format (current-error-port) "warning: choosing ~a~%" file))
     file))
 
 (define* (union-build output inputs