Melchior FRANZ:

- don't unlock an already unlocked mutex (Someone wanted to be on
  the safe side with this, but the result is undefined and makes
  pthread_mutex_destroy fail. Reference: manpage for
  pthread_mutexattr_gettype/The Open Group[1]: "Attempting to
  unlock a mutex of this type which is not locked results in
  undefined behaviour.")

- re-enabled all subsystem destructors again (this has been disabled
  because fgfs hung on exit, due to the mutex destroy failure from
  above.)


Reference:
  http://www.opengroup.org/onlinepubs/007908799/xsh/pthread_mutexattr_gettype.html

simgear/structure/subsystem_mgr.cxx

@@ -224,8 +224,7 @@ SGSubsystemGroup::Member::Member (const Member &)
 
 SGSubsystemGroup::Member::~Member ()
 {
-                                // FIXME: causes a crash
-//     delete subsystem;
+    delete subsystem;
 }
 
 void

simgear/threads/SGQueue.hxx

@@ -183,7 +183,7 @@ public:
     /**
      * Destroy this queue.
      */
-    ~SGBlockingQueue() { mutex.unlock(); }
+    ~SGBlockingQueue() {}
 
     /**
      *