Portability part2

But any test other than ``file_writer_test`` which tries to include ``writer.c`` will fail to compile, with a very mysterious message ``undefined symbol: CFENGINE_TEST_fwrite``. This is why I think the macro should be local to that test.

We can of course leave the ``CFENGINE_TEST`` macro in ``test.h`` for more general things, but I don't that macro should decide whether to use the fwrite hack.