Back in December, 2008, I posted Proper way to turn on SHMConfig in X Server 1.5 with input hotplugging, that was when HAL had not been deprecated. I checked my un-merging log, HAL was removed from my system in December, 2010, two years later of the post.

According to man synaptics, since X server 1.6, you don't need SHMConfig for changing settings on the fly via synclient. However, the -m touchpad activity monitoring option still requires SHMConfig, or you will get the same old error message:
Can't access shared memory area. SHMConfig disabled?
I have a plan for monitoring activity of touchpad, so I tried to turn it on again and the proper way is to edit /usr/share/X11/xorg.conf.d/50-synaptics.conf:
Section "InputClass"
        Identifier "touchpad catchall"
        Driver "synaptics"
        MatchDevicePath "/dev/input/event*"
        MatchIsTouchpad "on"
        Option "SHMConfig" "on"
The lines in bold text were what I added to it, the rest were the original content. I found out /dev/input/event* is crucial to get SHMConfig turned on. Without it, I still got the same error message saying SHMConfig disabled. I don't know if it also affects other options, but you don't actually need to put touchpad options here, you can change in your ~/.xinitrc or write a startup script.

If you don't have the file, then just create one in the same directory or one listed in man xorg.conf, X server looks into many different directories for configuration files.