Add a global variable ‘inNixShell’

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).

lib/trivial.nix

@@ -47,4 +47,7 @@ rec {
     readFile ../.version
     + (if pathExists suffixFile then readFile suffixFile else "pre-git");
 
+  # Whether we're being called by nix-shell.  This is useful to  
+  inNixShell = builtins.getEnv "IN_NIX_SHELL" == "1";
+
 }