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.
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.
Brad King