Fixed build error on VS2015

Author: JohanMabille

include/xtensor-blas/xblas_utils.hpp

@@ -195,7 +195,11 @@ namespace xt
      ***********************************/
 
     template <class T>
-    constexpr void assert_nd_square(const xexpression<T>& t) {
+#if !defined(_MSC_VER) || _MSC_VER >= 1910
+    constexpr
+#endif
+    void assert_nd_square(const xexpression<T>& t)
+    {
         auto& dt = t.derived_cast();
         if (dt.shape()[dt.dimension() - 1] != dt.shape()[dt.dimension() - 2])
         {