added check for unseq ExPolicy as it was added only in CXX20

libs/core/algorithms/include/hpx/parallel/algorithms/detail/generate.hpp

@@ -55,14 +55,19 @@ namespace hpx::parallel::detail {
     constexpr Iter sequential_generate_n_helper(
         Iter first, std::size_t count, F&& f, /*is unsequenced*/ std::true_type)
     {
+#ifdef HPX_WITH_CXX20_STD_EXECUTION_POLICES
         return std::generate_n(std::execution::unseq, first, count, f);
+#else
+        // do not use std::execution::seq, execution policies were added only in CXX17
+        return std::generate_n(first, count, f);
+#endif
     }
 
     template <typename Iter, typename F>
     constexpr Iter sequential_generate_n_helper(Iter first, std::size_t count,
         F&& f, /*is unsequenced*/ std::false_type)
     {
-        return std::generate_n(std::execution::seq, first, count, f);
+        return std::generate_n(first, count, f);
     }
 
     struct sequential_generate_n_t