HPCC-11319 System won't build without MySQL

Separate the flags for using the mysql repository and using the mysql plugin.

Signed-off-by: Richard Chapman <rchapman@hpccsystems.com>
Merge pull request #5801 from richardkchapman/nosql

HPCC-11319 System won't build without MySQL

Reviewed-by: Gavin Halliday <ghalliday@hpccsystems.com>