One or two things got moved around, but largely this is hooked up correctly
in the Unix configure system now: it can be dynamically loaded and fallback
gracefully if not available, or libXi can be directly linked to libSDL.
XInput support can be --disable'd from the configure script, too (defaults to
enabled).
Please note that while the framework is in place to gracefully fallback, the
current state of the source requires XInput. We'll need to adjust a few
things still to correct this.
5b27f96366