Improve support with MSVC

These fixes let all the tests to compile with the latest version of Microsoft Visual Studio Compiler (MSVC)

dune/istl/test/multirhstest.hh

@@ -40,7 +40,7 @@ template<typename T>
 struct Random {
   static T gen()
   {
-    return rand();
+    return T(std::rand())/RAND_MAX;
   }
 };