xvkbd: explicitly set LIBDIR and CONFDIR

In preparation to fix long-standing `imake` bug NixOS#135337 we need
to set `LIBDIR` and `CONFDIR` away from incorrectly embedded
`xorg-cf-files`. Before NixOS#135337 the package worked by chance.

Issue: NixOS#135337

pkgs/tools/X11/xvkbd/default.nix

@@ -13,9 +13,11 @@ stdenv.mkDerivation {
   buildInputs = [ libXt libXaw libXtst xorgproto libXi Xaw3d libXpm ];
   installTargets = [ "install" "install.man" ];
   makeFlags = [
-    "BINDIR=$(out)/bin"
-    "XAPPLOADDIR=$(out)/etc/X11/app-defaults"
-    "MANPATH=$(out)/man"
+    "BINDIR=${placeholder "out"}/bin"
+    "CONFDIR=${placeholder "out"}/etc/X11"
+    "LIBDIR=${placeholder "out"}/lib/X11"
+    "XAPPLOADDIR=${placeholder "out"}/etc/X11/app-defaults"
+    "MANPATH=${placeholder "out"}/man"
   ];
 
   meta = with lib; {