This matches a change in xorg-sgml-docs whereby the masterdb will look for
the target dbs into the same location as the generated documents.

The target dbs are now installed alongside the generated documents.
Previously they are installed in $prefix/sgml/X11/dbs alongside masterdb which
has the potential of installing outside the package prefix and cause
distcheck to fail when user does not have write permission in this package.

Requires XORG_CHECK_SGML_DOCTOOLS(1.8) which was released 2011-06-11