option doesn't affect innodb_plugin.

Currently it works as following:
* configure without relevant options producing two dynamic plugins
* configure --with-plugin-innodb (or legacy --with-innodb) producing
  static standard InnoDB
* configure --without-plugin-innodb (or legacy --without-innodb)
  disabling standard InnoDB
* configure --with-plugin-innodb_plugin producing dynamic innodb_plugin
* configure --without-plugin-innodb_plugin disabling innodb_plugin