[wwwdocs] Add some missing C++17 features

[Jonathan Wakely]

On 26/09/16 18:35 +0200, Jakub Jelinek wrote:
>Hi!
>
>The following patch adds some missing C++1z features to the table (wording
>from
>https://isocpp.org/std/standing-documents/sd-6-sg10-feature-test-recommendations#recs.cpp17
>except for P0386R2 which is too new for that (Jon, can you look up the
>feature test macro if any from newer SD-6 draft?).

The latest draft of SD-6 says __cpp_inline_variables >= 201606 for
that.

>Dunno what to do with P0400R0 that clang cxx_status.html lists too.

IMHO we don't need a separate entry for that.