Run makepkg in this directory, or see adig-pkgs to add the custom repository to have this listed by your pacman. Just run makepkg in the directory, or see github.com/adig-pkgs/adig-pkgs how to add the custom repo to have it listed by your pacman.

The headers are installed to:

/usr/local/include/ftxui

and the libraries: libcomponent.a, libdom.a, libscreen.a into:

/usr/local/lib/ftxui

You will get this message from makepkg too, don't forget to link to these libraries.