Add depencies on doxygen documentation to standalone documentation targets.

This seems to be needed for building pdfs.

[SVN r55729]

doc/Jamfile.v2

@@ -11,4 +11,5 @@ boostbook tribool
 	tribool.boostbook 
 	:
         <format>pdf:<xsl:param>boost.url.prefix=http://www.boost.org/doc/libs/release/doc/html
+        <dependency>reference
 	;