Could BSDSRCDIR vs. _SRC_TOP_ be checked in a way
that lets a setting for BSDSRCDIR be a sym-link
that overrides _SRC_TOP_ when they both agree?  As in

	.if defined(BSDSRCDIR) && $(BSDSRCDIR) != ""
	XX!= cd $(BSDSRCDIR) ; pwd
	YY!= cd $(_SRC_TOP_) ; pwd
	.if $(YY) != $(XX)
		Pitch a fit
	.endif
	_SRC_TOP_ = $(BSDSRCDIR)
	.else
	BSDSRCDIR = $(_SRC_TOP_)
	.endif

Paths in the face of sym-link games to stitch file
systems together will then "stay pretty."

Chris <jepeway@blasted-heath.com>.