Worked around the lack of a rand_r function on windows systems

unsupported/test/cxx11_eventcount.cpp

diff --git a/unsupported/test/cxx11_eventcount.cpp b/unsupported/test/cxx11_eventcount.cpp
index ebd019e..59039da 100644
--- a/unsupported/test/cxx11_eventcount.cpp
+++ b/unsupported/test/cxx11_eventcount.cpp
@@ -12,6 +12,14 @@
 #include "main.h"
 #include <Eigen/CXX11/ThreadPool>
 
+#ifdef EIGEN_COMP_MSVC_STRICT
+// Visual studio doesn't implementan rand_r() function since its
+// implementation of rand()is already thread safe
+int rand_r(unsigned int*) {
+  return rand();
+}
+#endif
+
 static void test_basic_eventcount()
 {
   std::vector<EventCount::Waiter> waiters(1);