gnome3.gnome-session: use absolute paths

TryExec needs absolute path too, otherwise the desktop file will be ignored
unless gnome-session is in PATH, in which case, we would not need to patch
Exec.

pkgs/desktops/gnome-3/core/gnome-session/default.nix

@@ -1,4 +1,4 @@
-{ fetchurl, stdenv, meson, ninja, pkgconfig, gnome3, glib, gtk, gsettings-desktop-schemas
+{ fetchurl, stdenv, substituteAll, meson, ninja, pkgconfig, gnome3, glib, gtk, gsettings-desktop-schemas
 , gnome-desktop, dbus, json-glib, libICE, xmlto, docbook_xsl, docbook_xml_dtd_412
 , libxslt, gettext, makeWrapper, systemd, xorg, epoxy }:
 
@@ -11,6 +11,15 @@ stdenv.mkDerivation rec {
     sha256 = "14nmbirgrp2nm16khbz109saqdlinlbrlhjnbjydpnrlimfgg4xq";
   };
 
+  patches = [
+    (substituteAll {
+      src = ./fix-paths.patch;
+      # FIXME: glib binaries shouldn't be in .dev!
+      gsettings = "${glib.dev}/bin/gsettings";
+      dbusLaunch = "${dbus.lib}/bin/dbus-launch";
+    })
+  ];
+
   mesonFlags = [ "-Dsystemd=true" ];
 
   nativeBuildInputs = [
@@ -29,15 +38,13 @@ stdenv.mkDerivation rec {
     patchShebangs meson_post_install.py
   '';
 
-  # FIXME: glib binaries shouldn't be in .dev!
   preFixup = ''
     for desktopFile in $(grep -rl "Exec=gnome-session" $out/share)
     do
       echo "Patching gnome-session path in: $desktopFile"
-      sed -i "s,^Exec=gnome-session,Exec=$out/bin/gnome-session," $desktopFile
+      sed -i "s,Exec=gnome-session,Exec=$out/bin/gnome-session," $desktopFile
     done
     wrapProgram "$out/bin/gnome-session" \
-      --prefix PATH : "${glib.dev}/bin" \
       --prefix GI_TYPELIB_PATH : "$GI_TYPELIB_PATH" \
       --suffix XDG_DATA_DIRS : "$out/share:$GSETTINGS_SCHEMAS_PATH" \
       --suffix XDG_DATA_DIRS : "${gnome3.gnome-shell}/share"\

pkgs/desktops/gnome-3/core/gnome-session/fix-paths.patch

@@ -0,0 +1,22 @@
+--- a/gnome-session/gnome-session.in
++++ b/gnome-session/gnome-session.in
+@@ -13,7 +13,7 @@
+   fi
+ fi
+ 
+-SETTING=$(gsettings get org.gnome.system.locale region)
++SETTING=$(@gsettings@ get org.gnome.system.locale region)
+ REGION=${SETTING#\'}
+ REGION=${REGION%\'}
+ 
+--- a/gnome-session/main.c
++++ b/gnome-session/main.c
+@@ -203,7 +203,7 @@
+         }
+         new_argv[i + 2] = NULL;
+         
+-        if (!execvp ("dbus-launch", new_argv)) {
++        if (!execvp ("@dbusLaunch@", new_argv)) {
+                 g_set_error (error, 
+                              G_SPAWN_ERROR,
+                              G_SPAWN_ERROR_FAILED,