The simple-scan package on SlackBuilds.org lists two dependencies: vala and libgusb. If you look at the README via sbopkg, it lists them as libgusb and vala. Order is important in this case. vala has to be installed first for simple-scan to compile properly as libgusb apparently has some vala hooks that simple-scan relies upon. If you build libgusb first and then vala, simple-scan's build will report an error (I didn't keep track of the exact error message, but it amounted to libgusb not having some sort of vala reference). Re-building libgusb resolved the issue for me.