sdb_plugin.m4: Provide more details if a plugin is disabled by default.
sdb_plugin.m4: Provide more details if a plugin is disabled by default.

The default value passed to AC_SDB_PLUGIN might contain additional information
about why the plugin has been disabled by default. This information should be
forwarded to the user.

tokkee/sysdb,sysdb/sysdb,tokkee/sysdb,sysdb/sysdb,tokkee/sysdb,sysdb/sysdb