Debian 9 "Stretch" está usando o driver libinput
$ mkdir -p /etc/X11/xorg.conf.d
$ echo 'Section "InputClass"
Identifier "libinput touchpad catchall"
MatchIsTouchpad "on"
MatchDevicePath "/dev/input/event*"
Driver "libinput"
Option "Tapping" "on"
EndSection' > /etc/X11/xorg.conf.d/40-libinput.conf
$ systemctl restart lightdm