Standard Macros

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.