NOTICES FOR USERS UPGRADING FROM EARLIER VERSIONS TO 6.x

As of 6.0, libpqxx requires C++11 or better.  Make sure that your libpqxx is
built against the same version of the C++ standard as your own application, or
there may be build problems.

It may be possible to paper over some mismatches.  If your application build
fails with errors about `std::experimental::optional`, try defining a macro
`PQXX_HIDE_EXP_OPTIONAL` in your application's build.  This will suppress
support for `std::experimental::optional` even if libpqxx was built to assume
that the feature is present.