Add "cxxstd" json field:

The "cxxstd" json field is being added to each Boost library's meta
json information for libraries whose minumum C++ standard compilation
level is C++11 on up. 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.

closes boostorg#2140

Author: eldiener

CHANGELOG.md

@@ -1,3 +1,7 @@
+* Add `cxxstd` to json field.
+
+--------------------------------------------------------------------------------
+
 Version 306:
 
 * Revert removal of lowest_layer_type from test stream.

meta/libraries.json

@@ -11,5 +11,6 @@
     ],
     "maintainers": [
         "Vinnie Falco <[email protected]>"
-    ]
+    ],
+    "cxxstd": "11"
 }