They created a language called simple (no longer under development)
This language is based on the source code of the Ring language
One of their goals was supporting FLTK and using it instead of Qt
So In theory, it can be done (In general a don't see a problem in supporting any library written in C or C++, It just need time and experience with these languages + knowledge about Ring API)
Would it require the Visual C compiler or could it be created with gcc/g++ ?"
We can use extensions developed using gcc/g++
If I remember correctly, RingQt itself did that in the past