irregularities with --no-builtin-variables

[Reto]

Hi,

I met some (for me) surprising result with the following makefile:

```
MAKEFLAGS += --no-builtin-rules
MAKEFLAGS += --no-builtin-variables

RM ?= hello
HAMSTER ?= world

all:
        echo ${RM}
        echo ${HAMSTER}
```

I'd expect that to print out "hello" and "world" (plus the usual command
repetition)

However I get this:
```
echo

echo world
world
```

Meaning the ?= assignment of RM was completely ignored, even though the
variable should not exists after --no-builtin-variables.

Is this a bug or am I missing something from the docs?

Kind regards,
Reto

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