If a variable has an empty value ifndef will enter the true branch.
> make EMPTY=0 all
echo anything
anything
> make EMPTY=1 all
makefile:12: *** variable is defined.. Stop.
> cat makefile
ifeq (1,${EMPTY})
VAR=
else
VAR=anything
endif
ifndef VAR
VAR=${error variable is defined.}
endif
all:
echo ${VAR}
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