This is primarily useful in Nix expressions that are also intended to
be used by nix-shell. It allows saying things like:
buildInputs = [ ... ] ++ (if inNixShell then [ emacs ] else [ ]);
This should not be used in Nixpkgs (since the question of what
constitutes a useful nix-shell environment is very subjective).
This makes it a bit easier to find out the current Nixpkgs version,
e.g.
$ nix-instantiate --eval '<nixpkgs>' -A lib.nixpkgsVersion
"14.02.35657.66f51a9"
