system: operating-system-etc-service: Set XCURSOR_PATH.

* gnu/system.scm (operating-system-etc-service): Set XCURSOR_PATH environment
variable so that libxcursor finds cursors in user and system profiles.
* gnu/packages/xorg.scm (libxcursor): Add a TODO for better handling of
XCURSOR_PATH in other profiles.

gnu/packages/xorg.scm

@@ -5300,6 +5300,15 @@ draggable titlebars and borders.")
         ("xproto" ,xproto)))
     (native-inputs
       `(("pkg-config" ,pkg-config)))
+;; TODO: add XCURSOR_PATH=.../share/icons to profile search paths, so
+;; libXcursor finds cursors installed into a profile.  If we solve bugs
+;; <http://bugs.gnu.org/20255> and <http://bugs.gnu.org/22138>, we can fix
+;; this with a search-path as follows:
+;;
+;;    (native-search-paths
+;;     (list (search-path-specification
+;;            (variable "XCURSOR_PATH")
+;;            (files '("share/icons")))))
     (home-page "https://www.x.org/wiki/")
     (synopsis "Xorg Cursor management library")
     (description "Xorg Cursor management library.")

gnu/system.scm

@@ -583,6 +583,9 @@ export INFOPATH=$HOME/.guix-profile/share/info:/run/current-system/profile/share
 export XDG_DATA_DIRS=$HOME/.guix-profile/share:/run/current-system/profile/share
 export XDG_CONFIG_DIRS=$HOME/.guix-profile/etc/xdg:/run/current-system/profile/etc/xdg
 
+# Make sure libXcursor finds cursors installed into user or system profiles.  See <http://bugs.gnu.org/24445>
+export XCURSOR_PATH=$HOME/.icons:$HOME/.guix-profile/share/icons:/run/current-system/profile/share/icons
+
 # Ignore the default value of 'PATH'.
 unset PATH