Introduce a portable EIGEN_SLEEP macro.

unsupported/test/cxx11_non_blocking_thread_pool.cpp

@@ -10,8 +10,8 @@
 
 #define EIGEN_USE_THREADS
 #include "main.h"
-#include <unistd.h>
 #include "Eigen/CXX11/ThreadPool"
+#include "Eigen/CXX11/Tensor"
 
 static void test_create_destroy_empty_pool()
 {
@@ -109,7 +109,7 @@ static void test_cancel()
   // Schedule a large number of closure that each sleeps for one second. This
   // will keep the thread pool busy for much longer than the default test timeout.
   for (int i = 0; i < 1000; ++i) {
-    tp.Schedule([]() { sleep(2); });
+    tp.Schedule([]() { EIGEN_SLEEP(2000); });
   }
 
   // Cancel the processing of all the closures that are still pending.