Attempted gcc 4.8 fix

include/fmt/ranges.h

@@ -356,6 +356,8 @@ struct formatter<TupleT, Char,
   };
 
  public:
+  FMT_CONSTEXPR formatter() { }
+
   FMT_CONSTEXPR void set_separator(basic_string_view<Char> sep) {
     separator_ = sep;
   }
@@ -451,6 +453,8 @@ struct range_formatter<
   }
 
  public:
+  FMT_CONSTEXPR range_formatter() { }
+
   FMT_CONSTEXPR auto underlying() -> detail::range_formatter_type<Char, T>& {
     return underlying_;
   }