[CMake] Setting up dependencies for non C++ files.

[Josef Karthauser]

We've got a load of 'shader' files which need to be compiled.  I've added custom targets for them, however like .cpp and .h files they have #include directives in as they can include other files.  Is there an easy way of utilising cmake's dependency checking stuff to get it to track these dependencies as if they were .h files?

Joe