.INCLUDE_DIRS question

[Martin d'Anjou]

I am try to come up with a working example of .INCLUDE_DIRS (new in make 
3.81), but I don't get it. Here is a snippet:

$(shell mkdir -p m && echo VAR=Hello > m/m.mk)

.INCLUDE_DIRS: m
include m.mk

all:
        echo $(VAR)

Is this the way it should be used? How should I set the search path for 
the make included files?

Thanks,
Martin