bug#12446: port better to POSIX hosts lacking _setjmp

[Paul Eggert]

On 09/15/2012 01:28 AM, Eli Zaretskii wrote:
> So what was the rationale of turning away from _setjmp

Some POSIXish hosts lack _setjmp, but have sigsetjmp,
and their longjmp implementations munge the signal masks.

>> > Messups are trickier, because they involve rare race conditions.
>> > They can occur if Emacs or a system call has set the signal mask to X,
>> > but a longjump unexpectedly changes it to Y, where Y is not equal to X.
> Why is that a messup?

Because Emacs then continues with the wrong sigmask, which can
cause signals to be blocked when they shouldn't be.