As described in http://www.nabble.com/Building-Modular-Xorg-t2596744.html#a7260015 a lot of xf86-input-* packages are breaked due to the new version of inputproto (1.4).
*** Bug 156659 has been marked as a duplicate of this bug. ***
*** Bug 156660 has been marked as a duplicate of this bug. ***
Affected packages are : xf86-input-calcomp xf86-input-citron xf86-input-dmc xf86-input-dynapro xf86-input-elo2300 xf86-input-microtouch xf86-input-penmount
I don't think a bug has ever been filed for this upstream. Please file a bug at https://bugs.freedesktop.org and post the link here. Assign it to or CC Daniel Stone, since he's taken responsibility for the break.