It shouldn't be too hard to port a C application. I don't know what kind of application it is. But I guess a great amount of the code doesn't need to be ported.
Search for those parts of the code which are platform-dependent and find out how the several platforms handle those functionalities. Perhaps some libraries are available for other platforms too.
I think it's a good idea to organise the code in such a way that platform-dependent code can be put apart. When going to a different platform you then only need to turn some compiler switches on or off.
