Creating a linux version should not be a big problem if you allready have a Mac version. The graphics and the sound code would be the about the same.
About creating a version usable on all linux distributions: You just need to ship the used libraries with the game and thats something you would do on all other platforms anyway. (And write some small 2 line shellscript to tell linux it should use the shipped libs)
For feature games you should may think about using SDL
for the actually system dependant code. (SDL is a free lib which provides things like window-initialisation, input handling, multithreading etc. in a platform independant way)