Use this where we are now using /etc/make.conf.
This allows people to override the current default of always including
/etc/make.conf. Setting __MAKE_CONF to /dev/null disables it
completely, while setting it to something else allows one to override
what is on the system. This can be desirable in situations where a
machine has many users and some of them want different defaults, or
defaults appropriate to cross building to be different than those for
normal building.
Not objected to by: arch@
0af93efb4d