treewide: move "extensions" drvs to dir

This cleans up the tree for pkgs/applications somewhat. Should not
change any hashes.

pkgs/applications/display-managers/lightdm/gtk-greeter.nix

@@ -0,0 +1,51 @@
+{ stdenv, fetchurl, lightdm, pkgconfig, intltool
+, hicolor-icon-theme, makeWrapper
+, useGTK2 ? false, gtk2, gtk3 # gtk3 seems better supported
+, exo
+}:
+
+#ToDo: bad icons with gtk2;
+#  avatar icon is missing in standard hicolor theme, I don't know where gtk3 takes it from
+
+let
+  ver_branch = "2.0";
+  version = "2.0.5";
+in
+stdenv.mkDerivation rec {
+  name = "lightdm-gtk-greeter-${version}";
+
+  src = fetchurl {
+    url = "${meta.homepage}/${ver_branch}/${version}/+download/${name}.tar.gz";
+    sha256 = "1pw70db8320wvkhkrw4i2qprxlrqy3jmb6yrr4bm3lgrizahiijx";
+  };
+
+  nativeBuildInputs = [ pkgconfig ];
+  buildInputs = [ lightdm exo intltool makeWrapper ]
+    ++ (if useGTK2 then [ gtk2 ] else [ gtk3 ]);
+
+  configureFlags = [
+    "--localstatedir=/var"
+    "--sysconfdir=/etc"
+  ] ++ stdenv.lib.optional useGTK2 "--with-gtk2";
+
+  NIX_CFLAGS_COMPILE = [ "-Wno-error=deprecated-declarations" ];
+
+  installFlags = [
+    "localstatedir=\${TMPDIR}"
+    "sysconfdir=\${out}/etc"
+  ];
+
+  postInstall = ''
+    substituteInPlace "$out/share/xgreeters/lightdm-gtk-greeter.desktop" \
+      --replace "Exec=lightdm-gtk-greeter" "Exec=$out/sbin/lightdm-gtk-greeter"
+    wrapProgram "$out/sbin/lightdm-gtk-greeter" \
+      --prefix XDG_DATA_DIRS ":" "${hicolor-icon-theme}/share"
+  '';
+
+  meta = with stdenv.lib; {
+    homepage = https://launchpad.net/lightdm-gtk-greeter;
+    platforms = platforms.linux;
+    license = licenses.gpl3;
+    maintainers = with maintainers; [ ocharles wkennington ];
+  };
+}