Makefile variables=, :=, ?=, +=

Makefile
Recursively expanded variables=
ex:
OPT=O2
var2=$(var1)
var1=$(OPT)

all:
echo "$(OPT) $(var1) $(var2)"

OPT, var1, var2 will be the same.
It will work fine.


Simply expanded variables:=
ex:
OPT=O2
var3:=(var2)
var2=$(var1)
var1=$(OPT)


all:
echo "$(OPT) $(var1) $(var2) $(var3)"

OPT, var1, var2 will be the same.
var3 will be empty.

Use the environment variable ?=

ex:
OPT=O2
var4?=DEBUG
var3:=(var2)
var2=$(var1)
var1=$(OPT)


all:
echo "$(OPT) $(var1) $(var2) $(var3) $(var4)"
OPT, var1, var2 will be the same.
var3 will be empty.
var4 will use the environment variable