Problem: nullptr is a reserved keyword in C++0x

Solution: use NULL instead to avoid build failure with older
compilers.

src/encoder.hpp

@@ -59,7 +59,7 @@ namespace zmq
         inline encoder_base_t (size_t bufsize_) :
             write_pos(0),
             to_write(0),
-            next(nullptr),
+            next(NULL),
             new_msg_flag(false),
             bufsize (bufsize_),
             in_progress (NULL)

src/socket_base.cpp

@@ -184,7 +184,7 @@ zmq::socket_base_t::socket_base_t (ctx_t *parent_, uint32_t tid_, int sid_, bool
     tag (0xbaddecaf),
     ctx_terminated (false),
     destroyed (false),
-    poller(nullptr),
+    poller(NULL),
     handle(NULL),
     last_tsc (0),
     ticks (0),

src/thread.hpp

@@ -55,8 +55,8 @@ namespace zmq
     public:
 
         inline thread_t ()
-            : tfn(nullptr)
-            , arg(nullptr)
+            : tfn(NULL)
+            , arg(NULL)
         {
         }
 

src/udp_engine.cpp

@@ -51,7 +51,7 @@ zmq::udp_engine_t::udp_engine_t() :
     fd(-1),
     session(NULL),
     handle(NULL),
-    address(nullptr),
+    address(NULL),
     send_enabled(false),
     recv_enabled(false)
 {