digikam: Add variant for external mysql database; rename variant for internal mysql database; update mysql dependency

[sk-public@…]

Support for external mysql databases has been disabled by default since digikam 4.4 (or thereabouts). This patch to the 4.8.0 portfile:

1. Adds a variant, mysql_external to enable support for external mysql databases.
2. Renames the current mysql_check variant to mysql_internal, to reflect what that option actually does -- enable the internal mysql database.
3. Updates the mysql dependency for the internal mysql database to version 5.6.

[sk-public@…]

Patch to digikam 4.8.0 for external mysql variant, rename current mysql_check

[ryandesign (Ryan Carsten Schmidt)]

What does it mean, "internal" and "external" MySQL databases?

Changing the dependency to "path:bin/mysql_config:mysql56" is not correct because the mysql56 port does not provide the file /opt/local/bin/mysql_config. Instead, you would just write "port:mysql56". But typically there should be variants allowing the user to choose which MySQL implementation they want: mysql55, mysql56, mariadb, mariadb-10.0, mariadb-10.1 or percona. The naming of these variants, especially the mariadb ones, has not yet been standardized. In each variant you will need to inform the configuration system somehow where that MySQL is located.