The "cxxstd" json field is being added to each Boost library's meta json
information for libraries in order to specify the minumum C++ standard
compilation level. The value of this field matches one of the values for
'cxxstd' in Boost.Build. The purpose of doing this is to provide
information for the Boost website documentation for each library which
will specify the minimum C++ standard compilation that an end-user must
employ in order to use the particular library. This will aid end-users
who want to know if they can successfully use a Boost library based on
their C++ compiler's compilation level, without having to search the
library's documentation to find this out.
