[cairo] Speeding up _cairo_fixed_from_double

[Robert O'Callahan]

> > If you're interested, there's a ad-hoc way of detecting IEEE754 from
> > other defined symbols (or by using a C99 symbol) found here:
> > http://sourceware.org/ml/automake/2003-05/msg00046.html
> 
> Thanks. So if we're really talking just "IBM mainframes and Cray",
> then I don't care about those.

For what it's worth, people run Firefox on S/390 machines. No, I don't
know why.

Rob