[DGD] MacOS installation directions

[Par Winzell]

Noah Gibbs wrote:
>   In DGD's README file, it says both to check "doc/Platforms" and
> "src/host/*/README".  In the case of MacOSX/Darwin, the former file is very
> helpful and the latter file, "src/host/mac/README" is misleading.  It's
> probably worth adding a note in the latter file that it applies only to MacOS 9
> and earlier, so that MacOS X doesn't claim it requires Symantec C 7.0.4 or
> newer :-)

Or rip out support for anything pre-X...

Zell