slim: Sort sessions

This ensures that xfce and most others DMs are preferred over
xterm. Previously slim used directory order, which is undefined.

Of course, it's just lucky that xfce < xterm lexicographically, but
that also applies to the other display managers, AFAIK. We should have
a way to specify a DM order, but that can be accomodated by generating
desktop entries like "<NN>-session.desktop".

Fixes #4300. Parenthetical to #12516.

pkgs/applications/display-managers/slim/default.nix

@@ -18,13 +18,17 @@ stdenv.mkDerivation rec {
       # slim's broken PAM session handling (see
       # http://developer.berlios.de/bugs/?func=detailbug&bug_id=19102&group_id=2663).
       ./run-once.patch
+
+      # Ensure that sessions appear in sort order, rather than in
+      # directory order.
+      ./sort-sessions.patch
     ];
 
   preConfigure = "substituteInPlace CMakeLists.txt --replace /lib $out/lib";
 
   cmakeFlags = [ "-DUSE_PAM=1" ];
 
-  NIX_CFLAGS_COMPILE = "-I${freetype}/include/freetype";
+  NIX_CFLAGS_COMPILE = "-I${freetype}/include/freetype -std=c++11";
 
   enableParallelBuilding = true;
 

pkgs/applications/display-managers/slim/sort-sessions.patch

@@ -0,0 +1,40 @@
+diff -ru -x '*~' slim-1.3.6-orig/cfg.cpp slim-1.3.6/cfg.cpp
+--- slim-1.3.6-orig/cfg.cpp	2013-10-02 00:38:05.000000000 +0200
++++ slim-1.3.6/cfg.cpp	2016-01-30 10:35:51.108766802 +0100
+@@ -14,6 +14,7 @@
+ #include <iostream>
+ #include <unistd.h>
+ #include <stdlib.h>
++#include <algorithm>
+ 
+ #include <sys/types.h>
+ #include <sys/stat.h>
+@@ -293,6 +294,8 @@
+ 
+ 	sessions.clear();
+ 
++        typedef pair<string,string> session_t;
++
+ 	if( !strSessionDir.empty() ) {
+ 		DIR *pDir = opendir(strSessionDir.c_str());
+ 
+@@ -325,7 +328,7 @@
+                                      }
+                              }
+                              desktop_file.close();
+-                             pair<string,string> session(session_name,session_exec);
++                             session_t session(session_name,session_exec);
+                              sessions.push_back(session);
+                              cout << session_exec << " - " << session_name << endl;
+                         }
+@@ -341,6 +344,10 @@
+         pair<string,string> session("","");
+         sessions.push_back(session);
+ 	}
++
++        std::sort(sessions.begin(), sessions.end(), [](session_t& a, session_t& b) -> bool{
++                return a.first < b.first;
++        });
+ }
+ 
+ pair<string,string> Cfg::nextSession() {