rtags: link to libclang.dylib at absolute path on darwin

pkgs/development/tools/rtags/default.nix

@@ -22,6 +22,12 @@ stdenv.mkDerivation rec {
 
   enableParallelBuilding = true;
 
+  postInstall = lib.optionalString stdenv.isDarwin ''
+    for f in $out/bin/{rc,rdm,rp}; do
+      install_name_tool -change @rpath/libclang.dylib ${llvmPackages.clang.cc}/lib/libclang.dylib ''${f}
+    done
+  '';
+
   meta = {
     description = "C/C++ client-server indexer based on clang";
     homepage = https://github.com/andersbakken/rtags;