Gettext wants to dlopen expat on "linux" platforms.  Guix used to fix
this, but the fix was removed for some reason in the past, resulting in
an xgettext that can't find expat and thus can't handle Glade files.
GTK+ 3.16.0 requires xgettext to support Glade files.  This patch fixes
the issue.  Thanks to Ludovic for the tip.