[PATCH v2] Let signbit use the builtin in C++ mode with gcc < 6.x (bug 22146)

[Gabriel F. T. Gomes]

On Mon, 16 Oct 2017, Joseph Myers wrote:

>This patch version is OK (with ChangeLog syntax fixed - missing "* " at 
>the start of the ChangeLog entry and missing blank line before the next 
>entry).

Just to confirm, this change does not require copyright assignment from
Romain, right?  (I don't know if Romain has it).