bug#26215: gschemas.compiled should not be added to the profile by multiple packages

["pelzflorian (Florian Pelz)" <pelzflorian@pelzflorian.de>]

Currently multiple packages contain the file
share/glib-2.0/schemas/gschemas.compiled (which is built by
glib-or-gtk-build-system). Doing so *works* (because each package’s
share directory in the Store is part of the XDG_DATA_DIRS environment
variable, GSettings looks for settings in each of the gschemas.compiled
files in the Store) but leads to *warnings* because only one package’s
gschemas.compiled can be added to the profile at the same time.

To avoid these misleading warnings, either
· no package’s gschemas.compiled should go to the profile on
  install *or*
· gschemas.compiled should not be created for each package by
  glib-or-gtk-build-system, instead it should be created only once
  in each profile by a profile hook from the GSettings data of all
  packages in the manifest,
· or something else?

This bug report follows a discussion here:
https://lists.gnu.org/archive/html/guix-devel/2017-03/msg00552.html

Is it easily possible to prevent a file from going from the Store to a
profile?

As for the other possible solution using a profile hook, John Darrington
asked:
> But what would happen if one had for example gnome-calculator in the
> system profile,
> and gnome-maps in the user profile?  Would it work under those
> circumstances?

A profile hook for gschemas.compiled would eliminate half the purpose of
glib-or-gtk-build-system I believe… It would still be used for setting
GTK_PATH GTK+ modules.