Some cleanups on the new XInput code. 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.