The package compiles OK in windows, even with 64 bits lazarus/fpc. It does not compile in linux, neither 32 nor 64 bits
My point was that the old Kylix compiler used those identifiers a lot, to manually figure out locale issues. Free Pascal is much more circumspect with such raw system imports since it has to work on multiple *nix OSes.
This is partially abstracted for Lazarus in units clocale and cwstring, and clocale defines at least nl_langinfo. Cwstring uses codeset internally, so that means that the information is maybe available somewhere in a portable way. (cwstrin is an unix specific "engine"/manager for unicode related stuff).
I quickly searched, and it seems to be used to fill
defaultsystemcodepage, so that might be what you want.