Changed build to generate a "html-synop.xsl" file and to have it

Changed build to generate a "html-synop.xsl" file and to have it
imported into the driver stylesheet. The build now uses the newly
added html2roff.xsl stylesheet to transform the ../xhtml/synop.xsl
file to generate the html-synop.xsl file. The only transformation
it currently does is to transform <br/> instances into line breaks
and to transform <pre></pre> instances into roff "no fill region"
markup.

This change is a cheap way to enable the manpages stylesheet to
correctly handle OO synopsis content, which it did not handle
correctly previously.