[Nix-dev] Darwin support

[Peter Simons]

Hi,

I committed a number of small changes today that make Nix work fairly
well in a Darwin environment. I managed to get all patches in without
triggering a stdenv re-build (or so I hope), but the Perl patch turned
out a little weird due to that objective. Anyway, the result seems to
work fine. I managed to install Emacs, Git, Subversion, Python, GHC, and
a bunch of other developer tools.

Take care,
Peter