Default to JSONSL_STATE_GENERIC mode without compile-time complaint.

The desire is to compile and link jsonsl without the need for -DJSONSL_STATE_GENERIC scattered around the build system.

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The jsonsl library is being used by a library that wants to be built conveniently for a variety of platforms, and linked statically or dynamically.  For convenience, JSONSL_STATE_USER_FIELDS ought to be opt-in, with JSONSL_STATE_GENERIC as default, without compile-time warnings.

- Nigel