PSA: Default C++ dialect is now C++17

[Marek Polacek via Gcc-patches]

As discussed last month:
<https://gcc.gnu.org/pipermail/gcc-patches/2020-May/545879.html>
it's time to change the C++ default to gnu++17.  I've committed the patch after
testing x86_64-pc-linux-gnu and powerpc64le-unknown-linux-gnu.  Brace 
yourselves!

Marek