fix a `configure' test

Thanks again to Tobias.
full unboxing for extflonums, plus `ffi/unsafe' support

Includes a repair for floating-point `min' and `max' that affects all
x86 builds that use SSE arithmetic, leaving the (otherwise unused)
floating-point stack in a bad state, which might have affected
x87-using C/foreign libraries running alongside Racket.
less noisy win32 builds