hmm. good luck...... however, wouldn't it be easier to write it in another cross platform language, like Python?
(and yet another option is to write cross platform code using C++, and a cross platform graphics engine (eg. X Window System, wxWindows, SDL, ...) The only changes you'd have to make for each platform is how to compile on that given platform, eg. c++ compiler on unix = g++, c++ on windows = cl.exe)