This can be achieved in an Xorg config file. First create the directory /etc/X11/xorg.conf.d
and copy 70-synaptics.conf
over from /usr/share/X11/xorg.conf.d/
sudo mkdir /etc/X11/xorg.conf.d
sudo cp /usr/share/X11/xorg.conf.d/70-synaptics.conf /etc/X11/xorg.conf.d/
Then change it to
Section "InputClass"
Identifier "touchpad catchall"
Driver "synaptics"
MatchIsTouchpad "on"
MatchDevicePath "/dev/input/event*"
Option "TapButton1" "1"
Option "TapButton2" "3" # multitouch
Option "TapButton3" "2" # multitouch
EndSection
This blog post explains the new layout of the X11 config files:
The /usr/share/X11/xorg.conf.d is for the Distro's and you are not
suppose to muck with stuff in there. Instead the user is suppose to put
custom files in /etc/X11/xorg.conf.d. You may need to create that directory. The .conf files in /etc/X11/xorg.conf.d run after the ones in
/usr/share/X11/xorg.conf.d so they control or override anything in /usr/share/X11/xorg.conf.d. Despite that I still usually number them a bit higher. So if I'm modifying the 50-wacom.conf in /usr/share/X11/xorg.conf.d I'll call the custom .conf file in /etc/X11/xorg.conf.d 52-wacom.conf. Maybe a bit of overkill.
For explanations on the available options, see the debian wiki