build: add entt library

include/entt/meta/fwd.hpp

@@ -0,0 +1,43 @@
+#ifndef ENTT_META_FWD_HPP
+#define ENTT_META_FWD_HPP
+
+#include "../stl/cstddef.hpp"
+#include "../stl/limits.hpp"
+
+namespace entt {
+
+struct meta_ctx;
+
+class meta_sequence_container;
+
+class meta_associative_container;
+
+class meta_any;
+
+class meta_handle;
+
+struct meta_custom;
+
+struct meta_data;
+
+struct meta_func;
+
+struct meta_base;
+
+class meta_type;
+
+template<typename>
+class meta_factory;
+
+/*! @brief Used to identicate that a sequence container has not a fixed size. */
+inline constexpr stl::size_t meta_dynamic_extent = (stl::numeric_limits<stl::size_t>::max)();
+
+/*! @brief Disambiguation tag for constructors and the like. */
+struct meta_ctx_arg_t final {};
+
+/*! @brief Constant of type meta_context_arg_t used to disambiguate calls. */
+inline constexpr meta_ctx_arg_t meta_ctx_arg{};
+
+} // namespace entt
+
+#endif