GUI development on Knoppix, Mac, Windows

[transistortoaster]

Hi,
I would like some input on what is the best package to develop a simple GUI that is platform-independent. I was thinking about some libraries that would sbe one layer above OpenGL, which is available to Linux, Mac OSX and Windows. There are just so many things to choose frome: Glui, glut, qt, ftlk, openml... I know Windows comes with opengl already inside. For the Mac's X11 I have to install opengl. I primarily want to develop with kdevelop and the Knoppix DVD. What its available that would fiot the requirements with or without a HD install of Knoppix.
Please give me your input.
Thanks,
Frank

[transistortoaster]

I just wanted to add that I am familiar with C, not C++
Frank

[transistortoaster]

The primary use of all this software will be to do 2D graphics, which will be signals for radio demodulation purposes.
Frank