Adding a dependency on Nepomuk to kdegraphics

[Aurélien Gâteau]

Hello,

I have been working on adding support for Nepomuk to Gwenview lately. I 
added the necessary CMake checks to find the required library, based on 
the kdebase checks.
I noticed Nepomuk is an optional dependency of kdebase. Am I right in 
assuming the dependency is optional because Nepomuk was not as available 
or stable as now when this dependency was introduced? Should I make 
kdegraphics dependency on Nepomuk optional too?

Aurélien