Fixed X11 XInput2 multitouch test to work in configure script.

configure wraps this C code in a main() function, so you can't declare a
function body in there. Besides, I'm not sure why we declared a function
that's part of the multitouch API anyhow.

Now we just reference a type that only exists if the headers have multitouch
support and call it a day.
Better attempt to detect available X11 XInput2 features.

Fixes Bugzilla #2306.