Please use copysign(1.0, fval) == 1.0 instead of your memcpy trick. It's
the cannonical way to check for negative zero. copysign() is always
available because we have our own implementation if the platform doesn't
provide one. We might also want to special case 1.0 and -1.0.

I've to check with Guido and Barry if we can get the optimization into
2.6.1 and 3.0.1. It may have to wait until 2.7 and 3.0.