[Issue 23583] ImportC: undefined identifier `__builtin___memmove_chk`

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=23583

--- Comment #2 from Walter Bright <bugzilla at digitalmars.com> ---
Looks like __GNUC__ will have to be #undef'd, too, at least for OSX.

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