Boost Config check for std::optional

[Tony E Lewis <TonyELewis@…>]

Please can you add a check for the availability of C++17's std::optional. I think this would be really helpful because:

• AFAIA, the major compilers all now have optional in std:: (rather than in std::experimental::)
• I get the impression that people are fast adopting std::optional as a standard part of their type vocabulary.
• I know at least one other ticket (ticket:12175) that's declining to support std::optional until Boost Config offers this feature test.

I'm not sure about Boost Config's policy about how it interacts with the standard's ​feature testing macros (which include one for std::optional). If you expect other libraries to use those feature tests directly, please can you point me to documentation for that policy that I can we can point other library devs to?

Thanks very much.

[John Maddock]

Moved to ​https://github.com/boostorg/config/issues/236