On X11, the flag Qt::WindowMinimizeButtonHint is ignored for window with a parent

On X11, a QDialog with a parent does not have the minimize button even with the flag Qt::WindowMinimizeButtonHint.

Here is an example to reproduce the problem:
#include <QtGui>

int main(int argc, char **argv){
QApplication app(argc, argv);
QMainWindow parent;
QWidget *dialog = new QWidget(&parent);
dialog->setWindowFlags(Qt::Dialog | Qt::WindowMinimizeButtonHint);
parent.show();
dialog->show();
return app.exec();
}

This problem is also reproductible with the Windowflags exemple in Qt.