Add 'third_party/nix/' from commit 'be66c7a6b24e3c3c6157fd37b86c7203d14acf10'

git-subtree-dir: third_party/nix
git-subtree-mainline: cf8cd640c1
git-subtree-split: be66c7a6b24e3c3c6157fd37b86c7203d14acf10

third_party/nix/shell.nix

@@ -0,0 +1,25 @@
+{ useClang ? false }:
+
+with import (builtins.fetchTarball https://github.com/NixOS/nixpkgs-channels/archive/nixos-19.03.tar.gz) {};
+
+with import ./release-common.nix { inherit pkgs; };
+
+(if useClang then clangStdenv else stdenv).mkDerivation {
+  name = "nix";
+
+  buildInputs = buildDeps ++ propagatedDeps ++ tarballDeps ++ perlDeps;
+
+  inherit configureFlags;
+
+  enableParallelBuilding = true;
+
+  installFlags = "sysconfdir=$(out)/etc";
+
+  shellHook =
+    ''
+      export prefix=$(pwd)/inst
+      configureFlags+=" --prefix=$prefix"
+      PKG_CONFIG_PATH=$prefix/lib/pkgconfig:$PKG_CONFIG_PATH
+      PATH=$prefix/bin:$PATH
+    '';
+}