diff --git a/pkgs/development/compilers/llvm/3.9/libc++/default.nix b/pkgs/development/compilers/llvm/3.9/libc++/default.nix
index da18e10c68a..bd2cc19d2e0 100644
--- a/pkgs/development/compilers/llvm/3.9/libc++/default.nix
+++ b/pkgs/development/compilers/llvm/3.9/libc++/default.nix
@@ -24,12 +24,13 @@ stdenv.mkDerivation rec {
     ../../libcxx-max_align_t.patch
   ];
 
-  buildInputs = [ cmake libcxxabi ] ++ lib.optional stdenv.isDarwin fixDarwinDylibNames;
+  nativeBuildInputs = [ cmake ];
+  buildInputs = [ libcxxabi ] ++ lib.optional stdenv.isDarwin fixDarwinDylibNames;
 
   cmakeFlags = [
-      "-DLIBCXX_LIBCXXABI_LIB_PATH=${libcxxabi}/lib"
-      "-DLIBCXX_LIBCPPABI_VERSION=2"
-      "-DLIBCXX_CXX_ABI=libcxxabi"
+    "-DLIBCXX_LIBCXXABI_LIB_PATH=${libcxxabi}/lib"
+    "-DLIBCXX_LIBCPPABI_VERSION=2"
+    "-DLIBCXX_CXX_ABI=libcxxabi"
   ] ++ stdenv.lib.optional stdenv.hostPlatform.isMusl "-DLIBCXX_HAS_MUSL_LIBC=1";
 
   enableParallelBuilding = true;