Disable spurious -Wstringop-overflow for setjmp/longjmp (bug 26647)

[Florian Weimer]

* Joseph Myers:

> Also, similar errors appear in the testsuite from __sigsetjmp calls in the 
> installed pthread.h, and presumably would apply in user programs that use 
> these pthread.h macros.  Should we apply a change like this one (with 
> appropriate version conditionals and comments; as an installed header, it 
> can't use libc-diag.h) to address such errors both for the glibc testsuite 
> and for user programs?  (With this and the other three patches I've posted 
> today, both glibc and its testsuite build OK for arm-linux-gnueabi.)

I do not think we can assume _Pragma support in this context.

Do we have to declare the first argument of __sigsetjmp as
struct __jmp_buf_tag __env[1]?

Thanks,
Florian
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