Mk/Uses/mysql.mk: Change the way of extracting version info from MySQL flavors
Use mysql_config --version instead of mysql --version because

in MySQL 8.0 the format of output is changed and it'd be [more reliable] and
easier to use just mysql_config because it only returns the numbers we want.

Reviewed by: brnrd, mat (mentor, portmgr)
Approved by: brnrd, mat (mentor, portmgr)
Sponsored by: EuroBSDCon Paris Devsummit
Differential Revision: https://reviews.freebsd.org/D12458