Pike doesn't build against libmysql (on MacOS)

Building Pike against libmysql v10 breaks due to the removal of several deprecated functions such as https://dev.mysql.com/doc/c-api/8.0/en/mysql-shutdown.html (page does not exist in the v10 docs). This is not a problem when building against libmariadb (as is standard on Debian and other Linuxes now).

Have not confirmed whether every deprecated function has a query equivalent (as mysql_shutdown does).

Testing against actual libmysql on Linux was unsuccessful for other reasons and is inconclusive, so this may be a Mac-specific issue?