cabal posix build-depends issue

[Malcolm Wallace]

Isaac Jones writes:
> > If it depends on posix, on a non-posix system, the user will get an
> > error because the posix package doesn't exist.
> > 
> > If it doesn't depend on posix on a posix-based system, then ghc won't
> > get the -package posix flag, and so cabal will fail to link.

There is a further problem: nhc98 has neither posix nor util packages,
so a straightforward dependency on either would be wrong.

Simon Marlow writes:
> My gut feeling is that we should be able to say something like
>   build-depends: if-not-os("mingw32",unix >= 1.0)

I would agree that we need platform-selected dependencies.  Platform
includes compiler, compiler version, OS, and possibly OS version.

Regards,
    Malcolm