I dunno, it seems like this is opening us to a new set of portability hazards (ie, sub-par implementations of arc4random) with not much gain to show for it.
Hence I reduced to three platforms only.
IIUC, what this code actually does is reseed itself from /dev/urandom every so often and work from a PRNG in between. That's not a layer that we need, because the code on top is already designed to cope with the foibles of /dev/urandom --- or, to the extent it isn't, that's something we have to fix anyway. So it seems like having this optionally in place just reduces what we can assume about the randomness properties of pg_strong_random output, which doesn't seem like a good idea.
