dovecot: changed to build ldap support as plugin

mail/dovecot/dovecot.conf

@@ -16,7 +16,7 @@
 var_append confopt ' ' "--enable-header-install"
 
 # package/option mappings for auto-configuration
-pkg_dovecot_options="openldap:ldap postgresql:pgsql mysql sqlite"
+pkg_dovecot_options="openldap:ldap=plugin postgresql:pgsql mysql sqlite"
 
 # automatic feature selection according to the package/option mappings
 pkg_dovecot_autoconfig() {