Only define S_IS... macros if they are not defined
Only define S_IS... macros if they are not defined

Even for non-MinGW builds, the macros may already be defined (e.g., by
other code in the project using luv).

Defining them whenever they do not exist handles both build types and
also avoids lots of compiler warnings when they already exist in
non-MinGW environments.

zhaozg/luv,luvit/luv,luvit/luv,zhaozg/luv