To address a thread related crash reported in osgParticle changed the ParticleSystem

to have an optional compile path for either using a OpenThreads::ReadWriteMutex or an 
OpenThreads::Mutex as it's base implementation, with the Mutex now being used by default.

src/osgParticle/ConnectedParticleSystem.cpp

@@ -116,7 +116,7 @@ void ConnectedParticleSystem::drawImplementation(osg::RenderInfo& renderInfo) co
 {
     osg::State& state = *renderInfo.getState();
 
-    OpenThreads::ScopedReadLock lock(_readWriteMutex);
+    ScopedReadLock lock(_readWriteMutex);
 
     const Particle* particle = (_startParticle != Particle::INVALID_INDEX) ? &_particles[_startParticle] : 0;
     if (!particle) return;