From the docs:
QApplication::QApplication ( int &
argc, char ** argv, bool GUIenabled )
Constructs an application object with
argc command line arguments in argv.
If GUIenabled is TRUE, a GUI
application is constructed, otherwise
a non-GUI (console) application is
created.
Set GUIenabled to FALSE for programs
without a graphical user interface
that should be able to run without a
window system.
You get that message because the compiler probably binds against this constructor method
QApplication::QApplication(Display *dpy,HANDLE visual=0,HANDLE colormap=0)
treating your first zero as a NULL pointer to Display * (I guess this is a Display structure you can get from X11)