#! [syntax]

<condition> {

    <command or definition>

    ...

}

#! [syntax]

#! [0]

win32 {

    SOURCES += paintwidget_win.cpp

}

#! [0]

#! [1]

!win32 {

    SOURCES -= paintwidget_win.cpp

}

#! [1]

unix {

    SOURCES += paintwidget_unix.cpp

}

#! [2]

macx {

    debug {

        HEADERS += debugging.h

    }

}

#! [2]

#! [3]

macx:debug {

    HEADERS += debugging.h

}

#! [3]

#! [4]

win32|macx {

    HEADERS += debugging.h

}

#! [4]