complain if $(way) is not a member of $(WAYS). This will help stop me
shooting myself in the foot by building things for nonexistent ways.
# Expects: $(WAYS) the possible "way" strings to one of
# which $(way) will be set
+ifneq "$(way)" ""
+ifeq "$(findstring $(way), $(WAYS))" ""
+$(error Unknown way $(way))
+endif
+endif
# So how does $(way) ever get set to anything? Answer, we recursively
# invoke make, setting $(way) on the command line.