[Interest] Qt5: Where's Q_OS_X11?

[Nikos Chantziaras]

Even though Q_OS_X11 is mentioned in the docs:

   "For example, Q_OS_X11 which is defined for the X Window System."

in:

http://doc-snapshot.qt-project.org/qt5-dev/qtcore/qtglobal.html#details

Q_OS_X11 is in fact not defined anywhere.  How do I find out at 
compile-time whether I'm building for an X11-based Qt version? 
Q_OS_UNIX doesn't mean X11 (could be Wayland, or who knows what else.)