>> I wonder if <sys/sysctl.h> is the right place - the macro isn't
>> sysctl-specific.  That said, I'm not sure where else.  <sys/param.h>
>> seems like overkill.
>
>types.h or one of the inttypes variants under it?

sys/cdefs.h?  i'm not a standards expert, but i do know that
sys/sysctl.h isn't (strictly) governed by standards, so putting it
there shouldn't violate anything...

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