append assignment operator in target specific variable

[Hyunho Cho]

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1. "foo" is simple variable.
   so result have to be 100 but is 200

foo :=
val := 100

all : foo += $(val)
all :
        @echo foo : $(foo)

val := 200

result is : 200
-------------------------------------------------------

2. If i change '+=' operator to ':=' then result is 100

foo :=
val := 100

all : foo := $(val)
all :
        @echo foo : $(foo)

val := 200

result is : 100
----------------------------------

foo :=  
val := 100
foo += $(val)
val := 200
$(info $(foo))

result is : 100