#2 Makefile should be OS agnostic.

The supplied Makefile comes pre-configured with
-D__LINUX__ in it. This is because there apparently is
no predefined compiler macro to identify whether or not
it's compiling under Linux. I know there has to be
some kind of predefined compiler macro for this purpose.

The Makefile should be OS agnostic, even if the
compiler macro can't be depended upon. Suitable
documentation should be written to explain it if necessary.