build: add EnTT library

include/entt/signal/fwd.hpp

@@ -0,0 +1,46 @@
+#ifndef ENTT_SIGNAL_FWD_HPP
+#define ENTT_SIGNAL_FWD_HPP
+
+#include "../stl/memory.hpp"
+
+namespace entt {
+
+template<typename>
+class delegate;
+
+template<typename = stl::allocator<void>>
+class basic_dispatcher;
+
+template<typename, typename = stl::allocator<void>>
+class emitter;
+
+class connection;
+
+struct scoped_connection;
+
+template<typename>
+class sink;
+
+template<typename Type, typename = stl::allocator<void>>
+class sigh;
+
+/*! @brief Alias declaration for the most common use case. */
+using dispatcher = basic_dispatcher<>;
+
+/*! @brief Disambiguation tag for constructors and the like. */
+template<auto>
+struct connect_arg_t {
+    /*! @brief Default constructor. */
+    explicit connect_arg_t() = default;
+};
+
+/**
+ * @brief Constant of type connect_arg_t used to disambiguate calls.
+ * @tparam Candidate Element to connect (likely a free or member function).
+ */
+template<auto Candidate>
+inline constexpr connect_arg_t<Candidate> connect_arg{};
+
+} // namespace entt
+
+#endif