Deprecate not defining NO_IMPLICIT_EXTERN_C

[Nathan Sidwell]

On 03/21/2018 05:28 PM, Joseph Myers wrote:

 > My list from 2005 of targets not defining NO_IMPLICIT_EXTERN_C then is
 > <https://gcc.gnu.org/ml/gcc/2005-05/msg00377.html>.

Thanks for the link.  I see you floated inverting the sense of the 
switch -- which would be good, as it appears we still can't remove it.

 > I think this should become a target hook, where the default is
 > implicit_extern_c returning false and only AIX (and any other OSes
 > requiring it) sets the hook to return true.

Yeah, I guess target hook might be the way to go these days.

Anyway, clearly a post-gcc8 thing.

nathan

-- 
Nathan Sidwell