How to check if feature (JSON) exists, and/or version
/*!50700 ... */ says:  If you are running a version older than 5.7.0, this is a comment.  Else it is sql.  It can be entire statements or fragments of statements.

Example (taken from mysqldump output):

/*!40103 SET @OLD_TIME_ZONE=@@TIME_ZONE */;


Presumably, @@TIME_ZONE was implemented in 4.1.3.

(Note:  I think the two spaces are part of the syntax.)