Emulate three button mouse

[Joe Riel]

How do I configure xfce4 to emulate a three button mouse?

Once I'm in X, I can manually use xinput to configure it, but
would prefer that it was automatic.  I tried adding a file
to /usr/share/X11/xorg.xconf.d:

$ cat /usr/share/X11/xorg.conf.d/80-middle-mouse-button.conf 
Section "InputClass"
     Identifier "middle button emulation class"
     MatchIsPointer "on"
     Option "Emulate3Buttons" "on"

however that does nothing.

-- 
Joe Riel