PATCH to enable testing C++17 by default

[Jeff Law]

On 10/17/18 1:31 PM, Marek Polacek wrote:
> As discussed in <https://gcc.gnu.org/ml/gcc-patches/2018-10/msg00752.html> it
> seems to be a high time we turned on testing C++17 by default.
> 
> The only interesting part is at the very end, otherwise most of the changes is
> just using { target c++17 } instead of explicit dg-options.  Removing
> dg-options has the effect that DEFAULT_CXXFLAGS comes in play, so I've removed
> a bunch of stray semicolons to fix -Wpedantic errors.
> 
> I wonder if we also want to enable 2a, but the overhead could be too much.  Or
> use 2a instead of 17?
> 
> Bootstrapped/regtested on x86_64-linux, ok for trunk?
> 
> 2018-10-17  Marek Polacek  <polacek@redhat.com>
> 
> 	* g++.dg/*.C: Use target c++17 instead of explicit dg-options.
> 	* lib/g++-dg.exp: Don't test C++11 by default.  Add C++17 to
> 	the list of default stds to test.Given this follows Jason's recommendations from the thread, OK for the
trunk.

I'll leave it up to Jason to decide when to add 2a and whether or not to
drop something at that time.

jeff