该代码目的是寻找到对应 版本的 i386-elf-objdump ，如果没有，则给出了其他的解决方法：修改
Makefile 里面的 GCCPREFIX 参数之后再进行 make；如果是为了关闭这个报错，则通过 gmake
GCCPREFIX=... 修改相关参数
其中 objdump 用于显示对象文件的信息
3. 调整 qemu 参数
该代码的目的是为了加载合适 qemu 硬件模拟器，要求为 qemu-system-i386 ，当对应目标不存在时，则
报错
4. 选择编译器和设置编译时的参数
GCCPREFIX := $(shell if i386-elf-objdump -i 2>&1 | grep '^elf32-i386$$' >/dev/null
2>&1; \
then echo 'i386-elf-'; \
elif objdump -i 2>&1 | grep 'elf32-i386' >/dev/null 2>&1; \
then echo ''; \
else echo "***" 1>&2; \
echo "*** Error: Couldn't find an i386-elf version of GCC/binutils." 1>&2; \
echo "*** Is the directory with i386-elf-gcc in your PATH?" 1>&2; \
echo "*** If your i386-elf toolchain is installed with a command" 1>&2; \
echo "*** prefix other than 'i386-elf-', set your GCCPREFIX" 1>&2; \
echo "*** environment variable to that prefix and run 'make' again." 1>&2; \
echo "*** To turn off this error, run 'gmake GCCPREFIX= ...'." 1>&2; \
echo "***" 1>&2; exit 1; fi)
endif
# try to infer the correct QEMU
ifndef QEMU
QEMU := $(shell if which qemu-system-i386 > /dev/null; \
then echo 'qemu-system-i386'; exit; \
elif which i386-elf-qemu > /dev/null; \
then echo 'i386-elf-qemu'; exit; \
elif which qemu > /dev/null; \
then echo 'qemu'; exit; \
else \
echo "***" 1>&2; \
echo "*** Error: Couldn't find a working QEMU executable." 1>&2; \
echo "*** Is the directory containing the qemu binary in your PATH" 1>&2; \
echo "***" 1>&2; exit 1; fi)
endif
# eliminate default suffix rules
# 建立默认的 suffix 规则
.SUFFIXES: .c .S .h
# delete target files if there is an error (or make is interrupted)
# 当 make 被中断、或文件存在错误时，删除对应的文件
.DELETE_ON_ERROR:
# define compiler and flags
ifndef USELLVM
HOSTCC := gcc
# 默认使用gcc
HOSTCFLAGS := -g -Wall -O2
CC := $(GCCPREFIX)gcc
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