MariaDB 10.2 incorporates MySQL 5.7. MySQL 5.7.9 (the first GA release
of the series) introduced an informational field to the InnoDB redo log
header, which identifies the server version where the redo log files
were created (initialized, resized or updated), in
WL#8845: InnoDB: Redo log format version identifier.

The informational message would be displayed to the user, for example
if someone tries to start up MySQL 8.0 after killing a MariaDB 10.2 server.
In the current MariaDB 10.2 source code, the identifier string would
misleadingly say "MySQL 5.7.14" (using the hard-coded version number in
univ.i) instead of "MariaDB 10.2.3" (using the contents of the VERSION
file, the build system copies to config.h and my_config.h).

This is only a cosmetic change. The compatibility check is based on a
numeric identifier.

We should probably also change the numeric identifier and some logic
around it. MariaDB 10.2 should refuse to recover from a crashed MySQL 5.7
instance, because the redo log might contain references to shared tablespaces,
which are not supported by MariaDB 10.2. Also, when MariaDB 10.2 creates
an encrypted redo log, there should be a redo log format version tag that
will prevent MySQL 5.7 or 8.0 from starting up.