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

[Gabriel F. T. Gomes]

On Tue, 17 Oct 2017, Gabriel F. T. Gomes wrote:

>OK.  I'm testing on powerpc64le and then I'll check this patch in on
>Romain's behalf (with Romain as git author, as usual, and with the fixes to
>the ChangeLog entry).

Now committed with the changes mentioned above, and with the bug number
fixed in the commit headline.

I can backport this to the stable branch, but after some days, as
suggested in https://sourceware.org/ml/libc-alpha/2017-10/msg00166.html