When it boils down to it, you probably don''t have to make too many API calls anyway. Most of them should just be in main.cpp. Really all you need to do is open a window, do controls, read time, and point to all the functions you going to need. It won''t be too hard to make a few of these main.cpp''s if you have a good knowledge of them and the seperate platforms. If you need the API for anything else, then make another library removed from the rest of the program so you can recode that in another API too.

OpenGL should be as far removed as possible from the rest of the code for easy conversion too. All the other libraries you create, like AI, maths, levels, etc could be done in relatively bog-standard C or C++ and shouldn''t need much changing.