buildworld: specifying an alternate make.conf

Hello,

to my understanding of man make and make.conf, which admittedly is very 
limited, I should be able to specify an alternate make.conf with this 
syntax:

# make -D __MAKE_CONF=<PATH>/make.conf buildworld

Now, since this looks somewhat strage - nowhere else I have found make 
variables with double underscores - and before wasting hours of 
compiling, I am wondering, is above correct? Or did I miss[understand] 
anything?

Thanks

Ede