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Some operation system, say Windows, use EXTENDED PRECISION by default.
(64 bits long double instead of 53 bits double)

Example, from my Win7 laptop:

Python return the correct rounded sum:
(thus *all* Python import modules also does 53-bits float rounding)

>>> 1e16 + 2.9999           # good way to test float precision setting
10000000000000002.0  # Python got the rounding right

Lua 5.4.0 (work1)  Copyright (C) 1994-2018, PUC-Rio
> string.format('%.17g\n', 1e16 + 2.9999)
10000000000000004    -- wrong, rounding in long double setting

For 53 bits double, to make same math result across platform, 
maybe add this to Lua.c ? (even if not needed)

fesetenv (FE_PC53_ENV);