Added the ability to switch to the new thread pool with a #define

unsupported/Eigen/CXX11/src/Tensor/TensorDeviceThreadPool.h

diff --git a/unsupported/Eigen/CXX11/src/Tensor/TensorDeviceThreadPool.h b/unsupported/Eigen/CXX11/src/Tensor/TensorDeviceThreadPool.h
index 41918eb..fc03d84 100644
--- a/unsupported/Eigen/CXX11/src/Tensor/TensorDeviceThreadPool.h
+++ b/unsupported/Eigen/CXX11/src/Tensor/TensorDeviceThreadPool.h
@@ -14,7 +14,11 @@
 
 // Use the SimpleThreadPool by default. We'll switch to the new non blocking
 // thread pool later.
+#ifdef EIGEN_USE_NONBLOCKING_THREAD_POOL
+typedef NonBlockingThreadPool ThreadPool;
+#else
 typedef SimpleThreadPool ThreadPool;
+#endif
 
 
 // Barrier is an object that allows one or more threads to wait until