configure test and requirements for using rapidjson

[John W. Eaton]

Hi,

Thanks for working on this new feature.

After

  changeset:   28623:5da49e37a6c9
  user:        Abdallah Elshamy <abdallah.k.elshamy@gmail.com>
  date:        Thu Aug 13 23:57:07 2020 +0900
  summary:     New functions jsondecode and jsonencode (bug #53100).

I noticed all tests for jsonencode and jsondecode were skipped on my 
system.  I have

  ii  rapidjson-dev  1.1.0+dfsg2-6 all          fast JSON 
parser/generator for C++ with SAX/DOM style API

installed.  When running configure, I see

  configure: WARNING: RapidJSON 1.1.0 or older found, but latest 
development version needed.  Octave will not be able to read or write 
JSON files.

The configure test that generates that message checks for 
rapidjson/cursorstreamwrapper.h, but the error message is about the 
RapidJSON version, and cursorstreamwrapper.h does not appear to be used 
directly in the Octave sources.  What feature(s) do we really need here? 
 If possible, think we should check for the required features and, if 
they are not found, issue a warning about that rather than something 
about a version since the need for a development version will likely 
change soon.

Thanks,

jwe