Double definition of errno?

[KJK::Hyperion]

Hi all. I'm not very familiar with C library implementations, so I'm 
wondering what's the purpose of a double definition of errno, both as 
"extern int" and as a macro, but, correct me if I'm wrong, it seems that 
some calls (namely, "system calls") are assumed to use the "extern int 
errno", while all the others will use the errno macro. I'd like to use an 
unified definition of errno, would this break something? (except the 
reentrant system calls provided by newlib, which I can override)