Hi Guix,

a couple of months ago we discussed how to make IBus and GTK themes work
across different GTK versions.

See:

  http://lists.gnu.org/archive/html/guix-devel/2015-09/msg00617.html

To summarise, GTK itself only supports path environment variables that
are recognised by *all* versions of GTK, such as GTK_EXE_PREFIX or
GTK_PATH.  This is a problem as modules built for GTK3 cannot be loaded
by GTK2 (and vice versa).  This can result in segfaults or other nasty
effects that are hard to debug.

The attached patches modify our gtk packages such that they apply
patches against the GTK sources to enable support for two new
environment variables:

  * GUIX_GTK2_PATH, which is honoured exclusively by gtk+ 2.x, and
  * GUIX_GTK3_PATH, which is only honoured by gtk+ 3.x

Directories specified in these variables are searched first when GTK
looks for modules, such as input method modules provided by IBus.  A
user would just set both of these variables such:

   export GUIX_GTK2_PATH=$HOME/.guix-profile/lib/gtk-2.0
   export GUIX_GTK3_PATH=$HOME/.guix-profile/lib/gtk-3.0

With these variables set GTK2 will find the IBus input methods for GTK2
(and not those for GTK3), and vice versa.

I rebuilt both GTKs in my profile, and rebuilt ibus, ibus-libpinyin, and
xfce4-terminal and after removing some cached state (a problem which
we’ll have to fix eventually) I could input Chinese characters on the
terminal, indicating that the environment variables are in fact
recognised.

An additional patch to ibus-libpinyin was required because it previously
only worked by accident as I had a package installed in my profile that
also propagated gtk+.  The third attached patch adds gtk-propagated
inputs to ibus-libpinyin for the sole purpose of adding their
girepository directories to the GI_TYPELIB_PATH with which we’re
wrapping some executables.