Patch to build libnl documentation out of tree

[Thomas Graf]

On Sat, Jul 30, 2011 at 04:20:01AM +0200, Gaudenz Steinlin wrote:
> Hi
> 
> While building the source code out of tree is fixed in git, the
> documentation is currently not buildable out of tree. The attached
> patch fixes this.

Applied.

I had to change the patch massively since Doxyfile.in has been modified
a lot since but I gave you credit in the commit description. Hope that
is fine.