thread safety warnings

[klaernie]

I recently fell into the problem, that JSON prefers JSON::XS, which is not thread safe. However using JSON::PP works perfectly fine.

Would it be a good idea for JSON to either:

• omit JSON::XS from the candidate list when loaded after threads
• warn that JSON::XS can cause issues in a threaded environment

I'd hope that this could prevent another person not reading the entire documentation of JSON::XS from getting into weird crashes and spending a few days hunting down what caused it, until finally figuring it out.

Best regards,
Andre