Fixed Graphviz support for versions older than 2.30.

DocGraphWriter now supports the two Graphviz libraries:
- libgraph (version < 2.30)
- libcgraph (version >= 2.30) 

Note: Versions older than 2.23 are unsupported.


git-svn-id: http://svn.gna.org/svn/etoile/trunk/Etoile/Developer/Services/DocGenerator@10184 7b3f36aa-e6db-49c1-8b4c-66eac1997e64