Disable touchpad acceleration on Debian Stretch (libinput, KDE Plasma)

For some unknown reason, there is no option exposed to disable acceleration when using a laptop touchpad with libinput. [insert gamer-oriented argument about how mouse acceleration is incompatible with muscle memory]

Thankfully it isn't too difficult to remove it in the source code and recompile libinput.

1. cd to a suitable empty folder
2. apt-get source libinput10
sudo apt build-dep libinput10
3. cd to the downloaded folder (e.g. cd libinput-1.6.3)
4. Edit the file src/filter.c, find:
   #defined TOUCHPAD_ACCELERATION 9.0
   And replace with:
   #defined TOUCHPAD_ACCELERATION 1.0
5. debuild -b -uc -us
6. Glance over the output to make sure build has succeeded
7. cd ..
sudo dpkg -i libinput10_1.6.3-1_amd64.deb
sudo dpkg -i libinput-bin_1.6.3-1_amd64.deb
(update version number as necessary)
8. Reboot