I'm not sure what all this `INTELPERC_INCLUDES` -> `INTELPERC_INCLUDE_DIR` -> `INTELPERC_INCLUDE_DIR_INTERNAL` dance is all about. What's wrong with just `find_file(INTELPERC_INCLUDE_DIR ...)`?

Also, the CMake convention for this sort of thing is `XXX_INCLUDE_DIRS` and `XXX_LIBRARIES` - in case multiple directories/libraries are needed in the future.