Fix SNAFU using $NIX_LDFLAGS_AFTER
Set MACOSX_DEPLOYMENT_TARGET

This variable sets the minimal Mac OS X version required for
running binaries produced by the Darwin toolchain. Since it
defaults to the version of the user's SDK, setting it explicitly
should make our builds more deterministic. It's now set to 10.6
because that's what hydra.nixos.org runs.