Before you start picking up a library like this, keep in mind that no immediate-mode GUI library I'm aware of (including this one from what I can see in the source code) has support for assistive technologies such as screen readers. To them the whole UI is one big black box, so your program is completely useless to anyone with a wide range of disabilities.
Do operation systems have APIs, so the program can send them what is on the screen? I mean just description like: There is the button with title "Push me" in the rectangle(50, 50, 100, 70), etc. Or are there any "screen reader" libraries?
On GNU/Linux, I think the only thing that exists in the Gtk+ features that were originally sponsored by Sun.