build: add EnTT library

include/entt/core/family.hpp

@@ -0,0 +1,35 @@
+#ifndef ENTT_CORE_FAMILY_HPP
+#define ENTT_CORE_FAMILY_HPP
+
+#include "../config/config.h"
+#include "fwd.hpp"
+
+namespace entt {
+
+/**
+ * @brief Dynamic identifier generator.
+ *
+ * Utility class template that can be used to assign unique identifiers to types
+ * at runtime. Use different specializations to create separate sets of
+ * identifiers.
+ */
+template<typename...>
+class family {
+    static auto identifier() noexcept {
+        static ENTT_MAYBE_ATOMIC(id_type) value{};
+        return value++;
+    }
+
+public:
+    /*! @brief Unsigned integer type. */
+    using value_type = id_type;
+
+    /*! @brief Statically generated unique identifier for the given type. */
+    template<typename... Type>
+    // at the time I'm writing, clang crashes during compilation if auto is used instead of value_type
+    inline static const value_type value = identifier();
+};
+
+} // namespace entt
+
+#endif