Transparency doesn't work in building Qt from sources

Hi,

I've encountered a problem in building a simple application using Qt 4.8.4 (built statically) and Linux Mint 14.

Code:

---

#include <QApplication>
#include <QLabel>
 
int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
 
    QLabel label("label");
    label.setAttribute(Qt::WA_TranslucentBackground);
    label.setFixedSize(400,400);
    label.show();
 
    return a.exec();
}

---

In my linux distribution I've enabled compositing and I've installed Qt Creator and the previous version of Qt libraries (4.8.3) from the repositories.
Using this version I don't have any problem with transparency, the label is shown as expected, but if I want to build statically my application using the 4.8.4 version, the background color is black.

I've tried to change the style using QApplication::setStyle(...) or using another Qt libraries version (4.8.2) built statically, but I was not able to solve the problem.
If anybody has any idea, please help me.
Thanks in advance