[darwin] - fix compilation with xcode6

As the title says - thx to @linusyang
@Memphiz Thanks for your efforts and you're correct. iOS 8 SDK wrapped up the `ntohll` function as a macro `__DARWIN_OSSwapInt64(x)` in `sys/_endian.h`. This makes many private implementations of `ntohll` fail to compile. So, adding a macro testing, such as `#ifndef ntohll`, can solve this issue.