Fix doc build for 2 libraries which use own css.

I changed the default to use doc/src/boostbook.css. So add an explicit
location to libraries which use their own stylesheet.

[SVN r64170]

doc/Jamfile.v2

@@ -24,6 +24,7 @@ boostbook standalone
         optional
     :
         <xsl:param>boost.root=../../../..
+        <xsl:param>html.stylesheet=boostbook.css
         <xsl:param>toc.max.depth=2
         <xsl:param>toc.section.depth=2
         <xsl:param>chunk.section.depth=1