Log Message:
Fix build of packages depending on mariadb55-client

When a package depends on mysql-client, in includes mk/mysql-buildlink3.mk
and may get buildlinked with mariadb55-client if MYSQL_VERSION_DEFAULT is
set to MARIADB55.

mariadb55-client's buildlink3.mk defined BUILDLINK_*.mariadb-client
variables, wich caused build problems with packages expecting