Question

Existing Qt applications should require no porting provided there is no
platform dependent code. Platform dependent code includes system calls,
calls to the underlying window system (Windows or X11), and Qt platform
specific methods such as QApplication::x11EventFilter().

For cases where it is necessary to use platform dependent code there are
macros defined that can be used to enable/disable code for each platform
using #ifdef directives:

Platform	Macro
Qt/X11	Q_WS_X11
Qt/Windows	Q_WS_WIN
Qt/Embedded	Q_WS_QWS