Make QT_STYLE_OVERRIDE env variable available to release builds

This shouldn't be restricted to internal builds.  It's nice to be able
to enable fusion style by default instead of GTK, for example.

Change-Id: Icf9b4c990ddd1152b7444948c98717faff1c5ad6
Reviewed-by: Giuseppe D'Angelo <giuseppe.dangelo@kdab.com>
Reviewed-by: Samuel Rødal <samuel.rodal@digia.com>

src/widgets/kernel/qapplication.cpp

@@ -965,11 +965,7 @@ QStyle *QApplication::style()
         // Compile-time search for default style
         //
         QString style;
-#ifdef QT_BUILD_INTERNAL
         QString envStyle = QString::fromLocal8Bit(qgetenv("QT_STYLE_OVERRIDE"));
-#else
-        QString envStyle;
-#endif
         if (!QApplicationPrivate::styleOverride.isEmpty()) {
             style = QApplicationPrivate::styleOverride;
         } else if (!envStyle.isEmpty()) {