I suspect that's because of the somewhat haphazard definition of NAN_INLINE.  I think the easiest and also best (or at least most idiomatic) solution is to simply s/NAN_INLINE/inline/g.

NAN_INLINE evaluates to `inline`, `inline __attribute__((always_inline))` or `__forceinline`, depending on the compiler.  It should always be 'inline' to avoid violating the Single Definition Rule (because unlike what most people think, 'inline' means 'local linkage', not 'inline this function' - local linkage just makes it easier to inline a function.) 

Apropos always_inline/__forceinline, those attributes should be used very sparingly.  Deciding whether it's best to inline a function or not is a job best left to the compiler.  It has a much better picture of code size, register pressure, etc.