Log Message:
Implement loadable plugin support for arts and esound output.  This is to
avoid using build variable definitions, and to allow optional use of these
daemons on machines where binary packages are used.  esound is no longer
needed in this package by default.  OSS is kept in the package as it was
before, beeing the default output driver.  Bump PKGREVISION to 5.

The patch has been reviewed by wiz@, the maintainer.  It has also been sent
back to SDL authors in the hope that they will implement this functionality.

Closes PR pkg/21774 by Marc Recht, which requested the addition of arts as
an optional dependancy (not done).