all: overrides=<>
make -f varname-dot-makeoverrides.mk stage_1 VAR=value
stage_1: overrides=< VAR>
make -f varname-dot-makeoverrides.mk stage_2 .VAR=too
stage_2: overrides=< VAR .VAR>
make -f varname-dot-makeoverrides.mk stage_3
stage_3: overrides=< .VAR VAR>
exit status 0