diff --git a/pkgs/development/tools/castxml/default.nix b/pkgs/development/tools/castxml/default.nix
index 603b155ee4f9..aea94633bae3 100644
--- a/pkgs/development/tools/castxml/default.nix
+++ b/pkgs/development/tools/castxml/default.nix
@@ -17,6 +17,11 @@ stdenv.mkDerivation rec {
     sha256 = "1hjh8ihjyp1m2jb5yypp5c45bpbz8k004f4p1cjw4gc7pxhjacdj";
   };
 
+  cmakeFlags = [
+    "-DCLANG_RESOURCE_DIR=${llvmPackages.clang-unwrapped}"
+    "-DSPHINX_MAN=${if withMan then "ON" else "OFF"}"
+  ];
+
   buildInputs = [
     cmake
     llvmPackages.clang-unwrapped
@@ -25,11 +30,6 @@ stdenv.mkDerivation rec {
 
   propagatedbuildInputs = [ llvmPackages.libclang ];
 
-  preConfigure = ''
-    cmakeFlagsArray+=(
-     ${if withMan then "-DSPHINX_MAN=ON" else ""}
-  )'';
-
   # 97% tests passed, 96 tests failed out of 2866
   # mostly because it checks command line and nix append -isystem and all
   doCheck=false;