Each operating system must have a subdir here, with a Makefile
The first target of that Makefile must build a file "sysdep.o" in this
directory.
The "clean" target of that Makefile must undo all the efects of the
first target, and must remove "sysdep.o" in this dir.

Files "sysdep.h", "signalent.h" and "syscallent.h" must be present
inside the directory after invoking the first target of the Makefile.

See the file "backend.h" for description of backend interfaces, which
have to be provided by "sysdep.o".
