Problem: socket_base uses macros for mutex lock and unlock

Solution: add a lock guard that takes optional a mutex and use it

src/mutex.hpp

@@ -181,6 +181,33 @@ namespace zmq
         scoped_lock_t (const scoped_lock_t&);
         const scoped_lock_t &operator = (const scoped_lock_t&);
     };
+
+
+    struct scoped_optional_lock_t
+    {
+        scoped_optional_lock_t (mutex_t* mutex_)
+            : mutex (mutex_)
+        {
+            if(mutex != 0)
+                mutex->lock ();
+        }
+
+        ~scoped_optional_lock_t ()
+        {
+            if(mutex != 0)
+                mutex->unlock ();
+        }
+
+    private:
+
+        mutex_t* mutex;
+
+        // Disable copy construction and assignment.
+        scoped_optional_lock_t (const scoped_lock_t&);
+        const scoped_optional_lock_t &operator = (const scoped_lock_t&);
+    };
+
+
 }
 
 #endif