> 
> GCC 7 is the only compiler
> I'm aware of that has it.  I'm only using it in one place though
> so it wouldn't be hard to forego that.  But I'm using string_view a
> lot already so have to have that.
> 

I think that some dirty work will get you to where you want to be with a
lesser version of GCC. You can either try to use the experimental code for
string_view, and I am thinking of something along the lines of what is
discussed here:

https://skebanga.github.io/string-view/

and in particular the test-build this guy makes.

And what I think may even be better is to take the stdc++ library from
GCC 7 and dumping it on top of 6.3.0, build it as I said and try it.
The third alternative would be to make a separate stc++ library of your
own and throwing that into a module. Use the GCC 7 code for stdc++ and
make a separate build of it using GCC 6.3.0. Install it somewhere else.
When you build _your_code_ link against it, either statically or with
some careful use of -rpath or LD_LIBRARY_PATH. I am sure with some trials
you will get it right.

Circa 2000 I used to juggle libraries that came with the PathScale
compiler to work with PGI and GCC. It was a similar stunt, but with
ready-built libraries. The linking step is the key.

Good luck.