Modified build system for itemmodels

[Stephen Kelly]

Stephen Kelly wrote:

> If it is, then I'd resubmit my proposal to have:
> 
> include(ItemModelsConfigCommon.cmake)
> 
> inside ItemModelsConfig.cmake, and generate it in the CMakeLists.txt

And by the way I also think we've covered all the discussion points needed 
in this thread and it's time to make a decision and move on.

If include(...Common) is not ok with you, then I prefer the copy+paste 
approach instead of the macro approach you proposed, to facilitate moving 
on.

(As long as you maintain all the copypasta :) )

Thanks,

Steve.