native+osx makefile cleanup

* add missing $(AD) to osx $(LINK) invocation
* move osx build determination to native makefile
* move old libc test to native makefile
* set objcopy to "true" - it is superfluous for native no matter what the system is
* add some documentation to natives makefile
Merge pull request #674 from LudwigOrtmann/native_osx_make

native/osx makefile cleanup