add etc/shell.nix for nix/nixOS

etc/shell.nix

@@ -0,0 +1,40 @@
+# This provides a shell with all the necesarry packages required to run mach and build servo
+# NOTE: This does not work offline or for nix-build
+
+with import <nixpkgs> {};
+
+clangStdenv.mkDerivation rec {
+  name = "servo-env";
+
+  buildInputs = [
+    # Native dependencies
+    fontconfig freetype openssl libunwind
+    xlibs.libxcb x11
+
+    gst_all_1.gstreamer
+    gst_all_1.gst-plugins-base
+    gst_all_1.gst-plugins-bad
+
+    rustup
+
+    # Build utilities
+    cmake dbus gcc git pkgconfig which llvm autoconf213 perl yasm m4
+    (python3.withPackages (ps: with ps; [virtualenv pip dbus]))
+  ];
+
+  LIBCLANG_PATH = llvmPackages.clang-unwrapped.lib + "/lib/";
+
+  # Allow cargo to download crates
+  SSL_CERT_FILE = "${cacert}/etc/ssl/certs/ca-bundle.crt";
+
+  # Enable colored cargo and rustc output
+  TERMINFO = "${ncurses.out}/share/terminfo";
+
+  shellHook = ''
+    # Fix missing libraries errors (those libraries aren't linked against, so we need to dynamically supply them)
+    export LD_LIBRARY_PATH=${lib.makeLibraryPath [ xorg.libXcursor xorg.libXrandr xorg.libXi ]}
+    # Fix invalid option errors during linking
+    # https://github.com/mozilla/nixpkgs-mozilla/commit/c72ff151a3e25f14182569679ed4cd22ef352328
+    unset AS
+  '';
+}