gnu: xfce4-session: Fix xflock4.

* gnu/packages/patches/xfce4-session-fix-xflock4.patch: New file.
* gnu-system.am (dist_patch_DATA): Add it.
* gnu/packages/xfce.scm (xfce4-session)[source]: Add patch.

gnu-system.am

@@ -687,6 +687,7 @@ dist_patch_DATA =						\
   gnu/packages/patches/xf86-video-trident-remove-mibstore.patch	\
   gnu/packages/patches/xf86-video-vmware-glibc-2.20.patch	\
   gnu/packages/patches/xfce4-panel-plugins.patch		\
+  gnu/packages/patches/xfce4-session-fix-xflock4.patch		\
   gnu/packages/patches/xfce4-settings-defaults.patch		\
   gnu/packages/patches/xmodmap-asprintf.patch 			\
   gnu/packages/patches/zathura-plugindir-environment-variable.patch

gnu/packages/patches/xfce4-session-fix-xflock4.patch

@@ -0,0 +1,31 @@
+From cbb9c769316b4d32956a2c78aa01a38b473f0cfc Mon Sep 17 00:00:00 2001
+From: David Thompson <dthompson2@worcester.edu>
+Date: Fri, 30 Oct 2015 08:30:43 -0400
+Subject: [PATCH] xflock4: Do not override PATH with hardcoded value.
+
+The PATH "/bin:/usr/bin" may not be a valid search path on the user's
+machine.  The screen locking program may be in /usr/local/bin or
+elsewhere.  Distros that do not conform to the FHS, such as GuixSD and
+NixOS, will not have their executables in either location.  Thus, we
+simply leave PATH alone.
+---
+ scripts/xflock4 | 3 ---
+ 1 file changed, 3 deletions(-)
+
+diff --git a/scripts/xflock4 b/scripts/xflock4
+index ec4d05d..e7981ac 100644
+--- a/scripts/xflock4
++++ b/scripts/xflock4
+@@ -21,9 +21,6 @@
+ #  Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
+ #
+ 
+-PATH=/bin:/usr/bin
+-export PATH
+-
+ # Lock by xscreensaver or gnome-screensaver, if a respective daemon is running
+ for lock_cmd in \
+     "xscreensaver-command -lock" \
+-- 
+2.5.0
+

gnu/packages/xfce.scm

@@ -421,7 +421,10 @@ your system in categories, so you can quickly find and launch them.")
                                   "/src/" name "-" version ".tar.bz2"))
               (sha256
                (base32
-                "01kvbd09c06j20n155hracsgrq06rlmfgdywffjsvlwpn19m9j38"))))
+                "01kvbd09c06j20n155hracsgrq06rlmfgdywffjsvlwpn19m9j38"))
+              (patches
+               ;; See: https://bugzilla.xfce.org/show_bug.cgi?id=12282
+               (list (search-patch "xfce4-session-fix-xflock4.patch")))))
     (build-system gnu-build-system)
     (arguments
      '(#:configure-flags