How to abort makefile if variable not set?

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How could I abort a make/makefile execution based on a makefile’s variable not being set/valued?

I came up with this, but works only if caller doesn’t explicitly run a target (i.e. runs make only).

ifeq ($(MY_FLAG),)
abort:   ## This MUST be the first target :( ugly
    @echo Variable MY_FLAG not set && false
endif

all:
    @echo MY_FLAG=$(MY_FLAG)

I think something like this would be a good idea, but didn’t find anything in make’s manual:

ifndef MY_FLAG
.ABORT
endif

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