[Bug c++/67160] static_assert feature test macro

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67160

--- Comment #5 from Jonathan Wakely <redi at gcc dot gnu.org> ---
And the advantage of the feature-test macro is that users can test for that
specific feature independent of the __cplusplus value.

Of course if it's effected by -pedantic then that means it *isn't* supported
with -std=c++11 -pedantic so the check needs to be more complicated.

In any case, once Paolo says he's done here I'll update
https://gcc.gnu.org/projects/cxx1z.html :-)