Dictionary System Includes, main branch (2023.07.06.)

In order to try to solve ATLINFR-4996, taught the dictionary generation about system includes. Unfortunately none of these updates changed anything about that warning, but I thought these would still be useful to collect into our main branch. (As they should make our builds slightly more robust.)

Basically the include directories were split into 2 lists, and the way they are composed into the dictionary generation command can now deal with either of these lists possibly being empty. (The "normal" include directory list can never be empty technically, but the "system" include directory list can be.)

Also shortened the directory lists a little using $<REMOVE_DUPLICATES:...>, just to simplify things a little bit for the dictionary generator.