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Previous: Simplest case: A Makefile
Most if not all actions a Makefile needs to perform can be
controlled entirely by setting one or more standard macros.
In general, defining a macro to have a (non-trivial) body
enables a particular behavior, while omitting a macro or defining
it to have a trivial body (i.e. an empty definition) disables
that behavior. This way, a single Makefile can in principle
control many distinct bulid actions. Note that it is not
always a good idea to structure Makefiles this way, but this
flexibility exists to allow individual mission teams to lay
out their directories in the way they choose.