GLX is not enabled when building with -qt-xcb

Hi:

We have to build Qt5 on Linux with -qt-xcb, but if we are doing so, the GLX could never be enabled, because the way the qtbase/configure is written, the GLX can only be enabled and config test will only run if we build with system xcb lib, which we do not want to.

Is there any rationale behind this logic?

Here is my temporary change. I tried to move the following part of code outside of the if [ "$CFG_XCB" != "no" ] scope:
if compileTest qpa/xcb-glx "xcb-glx" $QMAKE_CFLAGS_XCB $QMAKE_LIBS_XCB; then
CFG_XCB_GLX=yes
QT_CONFIG="$QT_CONFIG xcb-glx"
fi

just to make sure glx config test will run. After the change, I can see the QGLWidget works fine on my Deb 7 machine.