[Issue 23867] ImportC: undefined identifier `__builtin_isnan`

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=23867

--- Comment #1 from Lance Bachmeier <lance at lancebachmeier.com> ---
Adding #define __builtin_isnan isnan to the .c file fixes this.

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