bug#60311: json-available-p: make dynamically correct for Windows

[Mattias Engdegård]

The implementation of `json-available-p`,

  (and (fboundp 'json-serialize)
       (condition-case nil
           (json-serialize t)
         (:success t)
         (json-unavailable nil))))

probably isn't quite right on Windows: `json-serialize` is pure so it will be 
called at compile time and the result, "true", used in the code (actually not 
even that since the result is never used). Thus, if libjansson could not be 
loaded during actual Emacs use (as opposed to when Emacs was built), this would 
never be detected and json-available-p would still return t.

The patch below should take care of this. Would someone please help testing it 
on Windows?

json-available-p.diff
Description: Binary data