constants: add O_NONBLOCK constant

It appears that it is defined unconditionally on all supported unixes.

fix #7867 #7855

Signed-off-by: Trevor Norris <trev.norris@gmail.com>

src/node_constants.cc

@@ -110,6 +110,10 @@ void DefineConstants(Handle<Object> target) {
   NODE_DEFINE_CONSTANT(target, O_DIRECT);
 #endif
 
+#ifdef O_NONBLOCK
+  NODE_DEFINE_CONSTANT(target, O_NONBLOCK);
+#endif
+
 #ifdef S_IRWXU
   NODE_DEFINE_CONSTANT(target, S_IRWXU);
 #endif