Detect operating system in Makefile.

Makefile

# Detect operating system in Makefile.
# Author: He Tao
# Date: 2015-05-30

OSFLAG 				:=
ifeq ($(OS),Windows_NT)
	OSFLAG += -D WIN32
	ifeq ($(PROCESSOR_ARCHITECTURE),AMD64)
		OSFLAG += -D AMD64
	endif
	ifeq ($(PROCESSOR_ARCHITECTURE),x86)
		OSFLAG += -D IA32
	endif
else
	UNAME_S := $(shell uname -s)
	ifeq ($(UNAME_S),Linux)
		OSFLAG += -D LINUX
	endif
	ifeq ($(UNAME_S),Darwin)
		OSFLAG += -D OSX
	endif
		UNAME_P := $(shell uname -p)
	ifeq ($(UNAME_P),x86_64)
		OSFLAG += -D AMD64
	endif
		ifneq ($(filter %86,$(UNAME_P)),)
	OSFLAG += -D IA32
		endif
	ifneq ($(filter arm%,$(UNAME_P)),)
		OSFLAG += -D ARM
	endif
endif

all:
	@echo $(OSFLAG)

This is my laptop information

Device name	DESKTOP-RK0Q93P
Processor	13th Gen Intel(R) Core(TM) i5-1340P   1.90 GHz
Installed RAM	16.0 GB (15.6 GB usable)
Device ID	140104E8-85AA-3D55-990E-5185A62E169B
Product ID	00425-00000-00002-AA904
System type	64-bit operating system, x64-based processor
Pen and touch	No pen or touch input is available for this display

And this is the output i am getting

C:\Users\Hecker>make
-D WIN32 -D IA32

Can someone tell me whats the issue ?

OS_MAKE_NAME="OS"
OS_NAME := $(shell uname -s | tr A-Z a-z)
OS_ARCH := $(shell uname -m | tr A-Z a-z)

os:
	@echo ""
	@echo ""
	@echo "--- $(OS_MAKE_NAME) ---"
	@echo "OS_NAME: $(OS_NAME)"
	@echo "OS_ARCH: $(OS_ARCH)"
	@echo ""

make os

--- OS ---
OS_NAME: darwin
OS_ARCH: arm64