From Alexander Sinditskiy, build fix to allow building against versions older than librsvg-2.36.2


git-svn-id: http://svn.openscenegraph.org/osg/OpenSceneGraph/branches/OpenSceneGraph-3.2@14324 16af8721-9629-0410-8352-f15c8da7e697
Merged librsvg-2.0>=2.35 version check from svn/trunk.


git-svn-id: http://svn.openscenegraph.org/osg/OpenSceneGraph/branches/OpenSceneGraph-3.2@14325 16af8721-9629-0410-8352-f15c8da7e697