Add generated config.hpp to FILE_SET

[steve-downey]

Add the generated config.hpp to a FILE_SET, as well as the output BASE_DIR and remove the target_include_directory calls, so the install and exported config will be correct and complete.

Current install works around the problem but is mixing cmake paradigms which will eventually cause confusion and problems. Using file_sets with type header is the current model, and is in line with how module interfaces work, when those are supported.