If the user wants to configure libinput locally (e.g. to tune
touchpad sensitivity) the advertised[*] mechanism to do this
upstream is by adding a file /etc/libinput/local-overrides.quirks
Add --sysconfdir option to the libinput configurePhase so that we
build libinput to look for the file there instead of in libinput's
store path
[*] can't quite claim it as "supported", but it's certainly the
mechanism that's publically described. See
https://wayland.freedesktop.org/libinput/doc/latest/device-quirks.html
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