sercmd: pass OS define at compile time

utils/sercmd/Makefile

@@ -27,7 +27,7 @@ endif #ifneq (,$(MAKECMDGOALS))
 
 # erase common DEFS (not needed)
 C_DEFS:=
-DEFS:= -DNAME='"$(NAME)"' -DSRNAME='"$(MAIN_NAME)"' -DVERSION='"$(RELEASE)"' \
+DEFS:= -DNAME='"$(NAME)"' -DSRNAME='"$(MAIN_NAME)"' -DVERSION='"$(RELEASE)"' -D__OS_$(OS) \
 		$(filter -D%HAVE -DARCH% -DOS% -D__CPU% -D__OS%, $(DEFS))
 
 # use proper libs (we can't rely on LIBS value since we might be called