bug#12446: port better to POSIX hosts lacking _setjmp

[Paul Eggert]

On 09/15/2012 02:44 AM, Eli Zaretskii wrote:
>> Some POSIXish hosts lack _setjmp, but have sigsetjmp,
> 
> How did those hosts succeed to compile Emacs before your changes, then?

They used setjmp, which meant they were slower and
had some (rare) race conditions that caused problems.

> longjmp restores the environment, including the signal mask, to what
> it was when setjmp was called.

No, sometimes it does, sometimes it doesn't.
It depends on the platform.  If you want the signal mask
restored portably, you have to do it some other way, by using
siglongjmp or pthread_sigmask or something like that.