[Issue 23614] ImportC: __int128 not supported

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=23614

--- Comment #3 from Jacob Carlborg <doob at me.com> ---
(In reply to Walter Bright from comment #2)
> Do you really need this function, or is it just something in the header?
> Does the header have an #if alternative path in it?

No, I see now that it's guarded with __SIZEOF_INT128__. Adding `#undef
__SIZEOF_INT128__` in "importc.h" works.

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