On the surface it looks like an attempt to avoid a collision with some symbols. However this is a good question. The original commit that introduced this change can be found here: https://github.com/andrewlow/node/commit/fa1b515d7ef41337ff6e27266a11ce9995fe6a7f 

Removing the #define / #undef seems to have no negative impact to the code. I'll remove from the commit.