Some Clang versions warn with `-Wdocumentation-unknown-command` about
the `\internal` and `\snippet` command tag names.

The SunPro compiler does not support `#pragma pack`.

Even though we disable warnings when building jsoncpp itself, including
its headers from other CMake sources may still cause warnings.
Additional work will be needed to port CMake to newer jsoncpp interfaces
while still supporting jsoncpp 1.0.0.  For now, just suppress the
markup.

* upstream-jsoncpp:
  jsoncpp 2017-08-27 (4cfae897)

Even though we disable warnings when building jsoncpp itself, including
its headers from other CMake sources may still cause warnings.  Update
the code to remove these -Wdocumentation warnings.

The header uses std::ostream, so it must be declared.

Add '-w' or equivalent flag on compilers supporting it.
Tell MSVC to use its lowest warning level inside jsoncpp sources.

Update json/json.h to account for our lack of autolink.h.  Update
json/config.h to include KWSys Large File Support configuration so that
consistent stream libraries are used (on AIX with XL).

Add a cm_jsoncpp_reader.h header to include the CMake-provided copy of
the json/reader.h header from CMake sources.

Use a subtree merge to place the upstream content in the
Utilities/cmjsoncpp directory.