Default location for docbook stylesheets in Gentoo

This patch adds the default location for the DocBook DSSSL stylesheets
in gentoo's package system to the configure script.

The package in question is app-text/docbook-dsssl-stylesheets.

I'll understand if we don't want to include the location for every
single distribution under the sun, but figured it's more help than
harm to include it.

Cheers
BJ