We have had this user-modifable DEPENDFILE variable forever that does nothing
relevant for the user since fmake always used '.depend'. Bmake
introduced the .MAKE.DEPENDFILE variable that can be modified to change
the name of '.depend'.
Prior to r284288, bsd.progs.mk was setting .MAKE.DEPENDFILE to allow
working incremental builds. This was modified most likely to not
conflict with the META MODE handling of .MAKE.DEPENDFILE as it has a lot
more special logic for that variable.
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