[plugins/MultiThreading] FIX problem at initialization of mTaskMutex.v_

The plugin is broken since PR sofa-framework#549 as it removes the initialization
of the mutex. Possible explaination is that v_ is implementation
dependent but the existing initialization was done incorrectly
using a (int).

Now use BOOST_DETAIL_SPINLOCK_INIT which insure that the initialization
is coherent with the implementation used.

(I also cannot prevent myself to remove the two ugly commented lines)

applications/plugins/MultiThreading/src/TaskSchedulerBoost.h

@@ -30,10 +30,6 @@
 #include <boost/thread.hpp>
 #include <boost/thread/mutex.hpp>
 #include <boost/thread/condition.hpp>
-//#include <boost/detail/atomic_count.hpp>
-
-//#include <boost/pool/singleton_pool.hpp>
-
 
 
 namespace sofa
@@ -165,7 +161,7 @@ namespace sofa
 
 
 
-			mutable boost::detail::spinlock		mTaskMutex;
+            mutable boost::detail::spinlock		mTaskMutex {BOOST_DETAIL_SPINLOCK_INIT};
 			Task*		mTask[Max_TasksPerThread];
 			unsigned			mTaskCount;								
 			Task::Status*	mCurrentStatus;