build: add EnTT library

include/entt/poly/fwd.hpp

@@ -0,0 +1,21 @@
+#ifndef ENTT_POLY_FWD_HPP
+#define ENTT_POLY_FWD_HPP
+
+#include "../stl/cstddef.hpp"
+
+namespace entt {
+
+// NOLINTNEXTLINE(cppcoreguidelines-avoid-c-arrays, modernize-avoid-c-arrays)
+template<typename, stl::size_t Len = sizeof(double[2]), stl::size_t = alignof(double[2])>
+class basic_poly;
+
+/**
+ * @brief Alias declaration for the most common use case.
+ * @tparam Concept Concept descriptor.
+ */
+template<typename Concept>
+using poly = basic_poly<Concept>;
+
+} // namespace entt
+
+#endif