https://www.ultimatepp.org/app$ide$Files$en-us.html
- here a note: "icpp" extension was a problem when trying to compile U++ outside of theide/umk. It is not a problem anymore as its use in core libraries is replaced by another mechanism and deprecated.
This one is just for completness, you will not need it for GUI libraries, this is IMO sort of U++ mechanism for tasks you are using Meson (?):