thunderbolt: cleanup

pkgs/os-specific/linux/thunderbolt/default.nix

···
     sha256 = "02w1bfm7xvq0dzkhwqiq0camkzz9kvciyhnsis61c8vzp39cwx0x";
   };
 
-  buildInputs = [
-    boost
-    cmake
-    pkgconfig
-    txt2tags
+  nativeBuildInputs = [ cmake pkgconfig txt2tags ];
+  buildInputs = [ boost ];
+
+  cmakeFlags = [
+    "-DUDEV_BIN_DIR=${placeholder "out"}/bin"
+    "-DUDEV_RULES_DIR=${placeholder "out"}/etc/udev/rules.d"
   ];
-
-  # These can't go in the normal nix cmakeFlags because $out needs to be
-  # expanded by the shell, not by cmake or nix.  $ENV{out} doesn't work right
-  # either; it results in /build/source/build//nix/store/blahblahblahblah/bin/
-  # TODO: use ${placeholder "out"} when possible.
-  #       See https://github.com/NixOS/nixpkgs/pull/37693
-  preConfigure = ''
-    cmakeFlags+=" -DUDEV_BIN_DIR=$out/bin"
-    cmakeFlags+=" -DUDEV_RULES_DIR=$out/etc/udev/rules.d"
-  '';
 
   meta = {
     description = "Thunderbolt(TM) user-space components";