The three switches VMS, CPM, and MSDOS select the operating system.
Only one of these switches should be set to 1 at any time. If you set
more than one of them to 1, you will get errors when you compile.
To date, only the termio.c (low level terminal I/O) and spawn.c
(subjob creation) contain code conditionalized on the operating system.
Adding a new operating system consists mostly of adding the appropriate
magic incantations to these two files.