Every widgetset on every platform have their spezial needness.
Eg. on Windows a button can not be colored, on Linux it is possible. You must take care of such small differents. One soloution is to use right components and this mean you must tests the first time on all platforms, all the used components.
In the post before, there is the hint for installing all cross-compiler. This is the best way to test it. And use a Sourcecontrol system like git, so you can share the code between the systems and compile/test it nativ if needed.