Improve Makefile to allow building 32-bit targets on Linux using GCC

Makefile

@@ -1,14 +1,39 @@
-CC=i386-elf-gcc
-ASM=i386-elf-as
-LD=i386-elf-ld
+.PHONY: all dirs clean proper iso
+
+UNAME=$(shell uname | tr a-z A-Z)
+ifneq ($(findstring LINUX, $(UNAME)),)
+  BUILD_OS=LINUX
+endif
+ifneq ($(findstring DARWIN, $(UNAME)),)
+  BUILD_OS=MACOSX
+endif
+ifeq ($(BUILD_OS),)
+  $(error $(UNAME) support has not yet been coded into this Makefile)
+endif
+
+
+ifeq ($(BUILD_OS),MACOSX)
+  CC=i386-elf-gcc
+  ASM=i386-elf-as
+  LD=i386-elf-ld
+  ASFLAGS=
+  LDFLAGS=
+  BSCTFLAGS=
+endif
+ifeq ($(BUILD_OS),LINUX)
+  CC=gcc
+  ASM=as
+  LD=ld
+  ASFLAGS=--32
+  LDFLAGS=-m elf_i386 -s
+  BSCTFLAGS:=$(LDFLAGS)
+endif
 
 GFLAGS=
 CCFLAGS=-m32 -std=c11 -O2 -g -Wall -Wextra -Wpedantic -Wstrict-aliasing
 CCFLAGS+=-Wno-pointer-arith -Wno-unused-parameter
 CCFLAGS+=-nostdlib -nostdinc -ffreestanding -fno-pie -fno-stack-protector
 CCFLAGS+=-fno-builtin-function -fno-builtin
-ASFLAGS=
-LDFLAGS=
 
 BOOTSECT_SRCS=\
 	src/stage0.S
@@ -23,30 +48,33 @@ BOOTSECT=bootsect.bin
 KERNEL=kernel.bin
 ISO=boot.iso
 
-all: dirs bootsect kernel
+all: iso
+iso: dirs $(ISO)
+proper: clean all
 
 dirs:
 	mkdir -p bin
 
 clean:
-	rm ./**/*.o
-	rm ./*.iso
-	rm ./**/*.elf
-	rm ./**/*.bin
+	rm -f ./**/*.o
+	rm -f ./*.iso
+	rm -f ./**/*.elf
+	rm -f ./**/*.bin
+	rm -f $(BOOTSECT) $(KERNEL)
 
 %.o: %.c
 	$(CC) -o $@ -c $< $(GFLAGS) $(CCFLAGS)
 
 %.o: %.S
 	$(ASM) -o $@ -c $< $(GFLAGS) $(ASFLAGS)
 
-bootsect: $(BOOTSECT_OBJS)
-	$(LD) -o ./bin/$(BOOTSECT) $^ -Ttext 0x7C00 --oformat=binary
+$(BOOTSECT): $(BOOTSECT_OBJS)
+	$(LD) -o ./bin/$(BOOTSECT) $^ $(BSCTFLAGS) -Ttext 0x7C00 --oformat=binary
 
-kernel: $(KERNEL_OBJS)
+$(KERNEL): $(KERNEL_OBJS)
 	$(LD) -o ./bin/$(KERNEL) $^ $(LDFLAGS) -Tsrc/link.ld
 
-iso: bootsect kernel
+$(ISO): $(BOOTSECT) $(KERNEL)
 	dd if=/dev/zero of=boot.iso bs=512 count=2880
 	dd if=./bin/$(BOOTSECT) of=boot.iso conv=notrunc bs=512 seek=0 count=1
 	dd if=./bin/$(KERNEL) of=boot.iso conv=notrunc bs=512 seek=1 count=2048