ITK_COMPUTER_MEMORY_SIZE and ITK_WRAP_DIMS should be provided by the user, probably in /etc/make.conf. 
However, I don't know how this is properly done, and, on top if this, where and how it should be documented. 

ITK_COMPUTER_MEMORY_SIZE is used to optimize the streaming filter implementation that can handle images that are larger then the main memory. 

 ITK_WRAP_DIMS sets the image dimensions for which the language bindings are created (only Python is currently esposed with the ebuild).