doodle to show standard luaconf.h with luaconf.local.h

[imp]

Summary

This might be easier. We have a bog-standard luaconf.h, except for a
have the changes to lua for LUA_FLOAT_INT64, however, so we always must
define that.

Sponsored by: Netflix

Test Plan

NOTE: This is what I was trying to describe earlier today, with a simplification I thought of driving home from dinner.

This might be better than a unified luaconf.h that might have merge conflicts when updating.

[emaste]

IMO something along these lines makes sense

[imp]

In D38207#868260, @emaste wrote:

IMO something along these lines makes sense

Should I tidy it up and commit, or ????

[imp]

Add copyright / license / #pragma once

[imp]

tweaks

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This was landed in different ways.