sigwait was added to POSIX in 1995; it is not in older POSIX versions
or XPG4 / XPG4.2. Thus it should be declared only if
__USE_POSIX199506; signal.h wrongly declares it for __USE_POSIX.
(sigwaitinfo is correctly conditioned on __USE_POSIX199309.) This
patch fixes the condition on the declaration of sigwait.
Tested for x86_64. Because of other namespace problems this does not
allow any conform/ XFAILs to be removed.
[BZ #21550]
* signal/signal.h (sigwait): Only declare if [__USE_POSIX199506].
a0f8b11520