Fix warnings in the JSON, SAX and SAXophoNit libraries and in the `neo_doxygen` tool

Fix all the warnings in the following directories:

* `/lib/json`
* `/lib/sax`
* `/lib/saxophonit`
* `/contrib/neo_doxygen`

Signed-off-by: Jean-Christophe Beaupré <jcbrinfo@users.noreply.github.com>
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