lightdm_gtk_greeter: fix configureFlags to not be one long string

For whatever reason, setting `configureFlagsArray` causes
the configure invocation to escape spaces between parameters:

configure flags: --disable-static --disable-dependency-tracking --prefix=/nix/store/4bn8dsr743dp0g5j020jf8why5hkld3p-lightdm-gtk-greeter-2.0.6 --localstatedir=/var\ --sysconfdir=/etc\ --disable-indicator-services-command\ --enable-at-spi-command=/nix/store/li9bsxl8qja8qw0pa70xd572gw5hqsdm-at-spi2-core-2.30.1/libexec/at-spi-bus-launcher\ \ --launch-immediately

This actually doesn't break the build (!) but it does break
lightdm pretty badly for me and it's not hard to imagine why :).

Instead, use configureFlags as normal but just append the command
in `preConfigure` directly, so we can preserve the space.

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

@@ -23,18 +23,21 @@ stdenv.mkDerivation rec {
   buildInputs = [ lightdm exo intltool makeWrapper hicolor-icon-theme ]
     ++ (if useGTK2 then [ gtk2 ] else [ gtk3 ]);
 
-  configureFlagsArray = [
+  configureFlags = [
     "--localstatedir=/var"
     "--sysconfdir=/etc"
     "--disable-indicator-services-command"
-    "--enable-at-spi-command=${at-spi2-core}/libexec/at-spi-bus-launcher  --launch-immediately"
   ] ++ stdenv.lib.optional useGTK2 "--with-gtk2";
 
+  preConfigure = ''
+    configureFlagsArray+=( --enable-at-spi-command="${at-spi2-core}/libexec/at-spi-bus-launcher --launch-immediately" )
+  '';
+
   NIX_CFLAGS_COMPILE = [ "-Wno-error=deprecated-declarations" ];
 
   installFlags = [
     "localstatedir=\${TMPDIR}"
-    "sysconfdir=\${out}/etc"
+    "sysconfdir=${placeholder "out"}/etc"
   ];
 
   postInstall = ''