prevent macro redefined for JSON_HAS_INT64 (#1673)

include/json/config.h

diff --git a/include/json/config.h b/include/json/config.h
index 7f6e243..6971fa6 100644
--- a/include/json/config.h
+++ b/include/json/config.h
@@ -122,7 +122,9 @@
 #endif                // if defined(_MSC_VER)
 using LargestInt = Int64;
 using LargestUInt = UInt64;
+#ifndef JSON_HAS_INT64
 #define JSON_HAS_INT64
+#endif // ifndef JSON_HAS_INT64
 #endif // if defined(JSON_NO_INT64)
 
 template <typename T>