[build2] Constraining the C++ standard

[lyrahgames]

Fabian Meyer <fabian.meyer at posteo.de> wrote:

> Just one note:  libcxxopts is already published on cppget
> 
> https://cppget.org/libcxxopts?q=cxxopt
> 
> I think I just did not port the tests because of the autoptr problem 
> you have. I only ported the examples.

Thank you so much for the notice. That is very nice.
Unfortunately, it seems that I have not been paying enough attention
and may have done some useless extra work.
If you would be interested, I could propose a pull request
to the already existing package repository to add some functionality of 
my trial.
Otherwise, I will archive my repository for documentation.