Fix BLR optimization (sorry)

In which I spend an hour and a half tracking down a bug where I wrote #ifdef instead of #ifndef.

(As I mentioned in the original pull request, stack overflow checking - and therefore, in the final version, using a custom stack - is disabled on Windows because the kernel is too opinionated about RSP.  Except I accidentally disabled it on non-Windows, at which point Windows started complaining because, even though the stack didn't actually overflow, in this version I removed the TIB stack limit adjustment so it thought it did.)