build: add EnTT library

include/entt/stl/functional.hpp

@@ -0,0 +1,55 @@
+#ifndef ENTT_STL_FUNCTIONAL_HPP
+#define ENTT_STL_FUNCTIONAL_HPP
+
+/*! @cond ENTT_INTERNAL */
+#if __has_include(<entt/ext/stl/functional.hpp>)
+#    include <entt/ext/stl/functional.hpp>
+#else
+#    include <functional>
+#    include <version>
+#    include "../config/config.h"
+
+namespace entt::stl {
+
+using std::equal_to;
+using std::function;
+using std::hash;
+using std::invoke;
+using std::less;
+
+} // namespace entt::stl
+
+#    ifndef ENTT_FORCE_STL
+#        if defined(__cpp_lib_ranges)
+#            define ENTT_HAS_IDENTITY
+namespace entt::stl {
+
+using std::identity;
+
+} // namespace entt::stl
+#        endif
+#    endif
+
+#    ifndef ENTT_HAS_IDENTITY
+#        include <utility>
+
+namespace entt::stl {
+
+struct identity {
+    using is_transparent = void;
+
+    template<typename Type>
+    [[nodiscard]] constexpr Type &&operator()(Type &&value) const noexcept {
+        return std::forward<Type>(value);
+    }
+};
+
+} // namespace entt::stl
+#    endif
+
+#endif
+/*! @endcond */
+
+#undef ENTT_HAS_IDENTITY
+
+#endif