https://devhints.io/makefile
很像shell的语法，如#表示注释
但还是有些区别的，如a = val的等号两边是可以有空格的
详细信息参照makefile 节选

Assignment

uglify = $(uglify)        # assignment
compressor := $(uglify)   # lazy assignment
prefix ?= /usr/local      # safe assignment

Magic variables

out.o: src.c src.h
  $@   # "out.o" (target)
  $<   # "src.c" (first prerequisite)
  $^   # "src.c src.h" (all prerequisites)

%.o: %.c
  $*   # the 'stem' with which an implicit rule matches ("foo" in "foo.c")

also:
  $+   # prerequisites (all, with duplication)
  $?   # prerequisites (new ones)
  $|   # prerequisites (order-only?)

  $(@D) # target directory

Find files

js_files  := $(wildcard test/*.js)
all_files := $(shell find images -name "*")

Includes

-include foo.make

Conditionals

foo: $(objects)
ifeq ($(CC),gcc)
        $(CC) -o foo $(objects) $(libs_for_gcc)
else
        $(CC) -o foo $(objects) $(normal_libs)
endif