Allow setting of MK_FOO by anybody. There are too many edge cases that

Allow setting of MK_FOO by anybody. There are too many edge cases that
we're preventing now with this policy. However, these edge cases
should be rare and all that set MK_FOO directly.
WITH_FOO and WITHOUT_FOO both being defined now result in the
non-default behavior happening silently. Users needing determinism
here fall into the edge case exception for MK_FOO setting.