Started to implement a portable way to yield.

unsupported/Eigen/CXX11/ThreadPool

diff --git a/unsupported/Eigen/CXX11/ThreadPool b/unsupported/Eigen/CXX11/ThreadPool
index 6cd057b..fe00a0b 100644
--- a/unsupported/Eigen/CXX11/ThreadPool
+++ b/unsupported/Eigen/CXX11/ThreadPool
@@ -45,9 +45,10 @@
 #include <functional>
 #include <memory>
 
+#include "src/ThreadPool/ThreadLocal.h"
+#include "src/ThreadPool/ThreadYield.h"
 #include "src/ThreadPool/EventCount.h"
 #include "src/ThreadPool/RunQueue.h"
-#include "src/ThreadPool/ThreadLocal.h"
 #include "src/ThreadPool/ThreadPoolInterface.h"
 #include "src/ThreadPool/ThreadEnvironment.h"
 #include "src/ThreadPool/SimpleThreadPool.h"

unsupported/Eigen/CXX11/src/ThreadPool/EventCount.h

diff --git a/unsupported/Eigen/CXX11/src/ThreadPool/EventCount.h b/unsupported/Eigen/CXX11/src/ThreadPool/EventCount.h
index 16eee1a..6dd64f1 100644
--- a/unsupported/Eigen/CXX11/src/ThreadPool/EventCount.h
+++ b/unsupported/Eigen/CXX11/src/ThreadPool/EventCount.h
@@ -81,7 +81,7 @@
       if (int64_t((state & kEpochMask) - epoch) < 0) {
         // The preceeding waiter has not decided on its fate. Wait until it
         // calls either CancelWait or CommitWait, or is notified.
-        std::this_thread::yield();
+        EIGEN_THREAD_YIELD();
         state = state_.load(std::memory_order_seq_cst);
         continue;
       }
@@ -112,7 +112,7 @@
       if (int64_t((state & kEpochMask) - epoch) < 0) {
         // The preceeding waiter has not decided on its fate. Wait until it
         // calls either CancelWait or CommitWait, or is notified.
-        std::this_thread::yield();
+        EIGEN_THREAD_YIELD();
         state = state_.load(std::memory_order_relaxed);
         continue;
       }

unsupported/Eigen/CXX11/src/ThreadPool/ThreadYield.h

diff --git a/unsupported/Eigen/CXX11/src/ThreadPool/ThreadYield.h b/unsupported/Eigen/CXX11/src/ThreadPool/ThreadYield.h
new file mode 100644
index 0000000..a859c7b
--- /dev/null
+++ b/unsupported/Eigen/CXX11/src/ThreadPool/ThreadYield.h
@@ -0,0 +1,20 @@
+// This file is part of Eigen, a lightweight C++ template library
+// for linear algebra.
+//
+// Copyright (C) 2016 Benoit Steiner <benoit.steiner.goog@gmail.com>
+//
+// This Source Code Form is subject to the terms of the Mozilla
+// Public License v. 2.0. If a copy of the MPL was not distributed
+// with this file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+#ifndef EIGEN_CXX11_THREADPOOL_THREAD_YIELD_H
+#define EIGEN_CXX11_THREADPOOL_THREAD_YIELD_H
+
+// Try to come up with a portable way to yield
+#if EIGEN_COMP_GNUC && EIGEN_GNUC_AT_MOST(4, 7)
+#define EIGEN_THREAD_YIELD() sched_yield()
+#else
+#define EIGEN_THREAD_YIELD() std::this_thread::yield()
+#endif
+
+#endif  // EIGEN_CXX11_THREADPOOL_THREAD_YIELD_H