= is the most basic assignment
: = is to overwrite the previous value
? = is to assign the value after the equal sign if it has not been assigned
+= is to add the value after the equal sign
The difference between = and := is that = will determine the value of the variable after the makefile is expanded, that is, the last specified value
eg：
x = foo
y = $(x) bar
x = xyz
In the example above, the value of y would be xyz bar , not foo bar .
And := indicates that the value of the variable is determined by its position in the makefile, not the final value after the expansion of the entire makefile.
In the above example, the value of y will be foo bar, not xyz bar