build: add entt library

include/entt/graph/fwd.hpp

@@ -0,0 +1,28 @@
+#ifndef ENTT_GRAPH_FWD_HPP
+#define ENTT_GRAPH_FWD_HPP
+
+#include "../core/fwd.hpp"
+#include "../stl/concepts.hpp"
+#include "../stl/cstddef.hpp"
+#include "../stl/memory.hpp"
+
+namespace entt {
+
+/*! @brief Undirected graph category tag. */
+struct directed_tag {};
+
+/*! @brief Directed graph category tag. */
+struct undirected_tag: directed_tag {};
+
+template<stl::derived_from<directed_tag>, typename = stl::allocator<stl::size_t>>
+class adjacency_matrix;
+
+template<typename = stl::allocator<id_type>>
+class basic_flow;
+
+/*! @brief Alias declaration for the most common use case. */
+using flow = basic_flow<>;
+
+} // namespace entt
+
+#endif