Compiler version specific configuration support added

src/include/units/bits/hacks.h

@@ -22,10 +22,13 @@
 
 #pragma once
 
+#include <units/bits/config.h>
 #include <experimental/ranges/concepts>
 
 namespace std {
 
+#ifndef UNITS_HAS_STD_TYPE_IDENTITY
+
   // type_identity
   template<typename T>
   struct type_identity { using type = T; };
@@ -33,6 +36,8 @@ namespace std {
   template<typename T>
   using type_identity_t = typename type_identity<T>::type;
 
+#endif // UNITS_HAS_STD_TYPE_IDENTITY
+
   // concepts
   using experimental::ranges::Same;
   using experimental::ranges::Integral;