What's the difference between := and = in Makefile?－Welkin小窩

Variables defined with ‘=’ are recursively expanded variables.

Variables defined with ‘:=’ or ‘::=’ are simply expanded variables.

(Both forms are equivalent in GNU make; however only the ‘::=’ form is described by the POSIX standard.)

:= (Simply Expanded Variable) The value is scanned for once and for all expanding any references to other variables and functions, when variable is defined. e.g.
x:=foo
y:=$(x) bar
x:=later
so above is equivalent to
y:=foo bar
x:=later
+= is used for appending more text to variables e.g.
objects=main.o foo.o bar.o
objects+=new.o
which will set objects to 'main.o foo.o bar.o new.o'
= is for recursively expanded variable.The value is install verbatim; if it contains reference to other variables these variables are expanded whenever this variable is substituted.And this is known as recursive expansion.