message: Fix build error with OSL or MDL disabled
diff: @@ -1413,6 +1413,7 @@ void Viewer::saveShaderSource(mx::GenContext& context)
  new ng::MessageDialog(this, ng::MessageDialog::Type::Information, "Saved ESSL source: ",
  sourceFilename.asString() + "_essl_*.glsl");
  }
  +#if MATERIALX_BUILD_GEN_OSL
  else if (context.getShaderGenerator().getTarget() == mx::OslShaderGenerator::TARGET)
  {
  mx::ShaderPtr shader = createShader(elem->getNamePath(), context, elem);
  @@ -1421,6 +1422,8 @@ void Viewer::saveShaderSource(mx::GenContext& context)
  writeTextFile(pixelShader, sourceFilename);
  new ng::MessageDialog(this, ng::MessageDialog::Type::Information, "Saved OSL source: ", sourceFilename);
  }
  +#endif
  +#if MATERIALX_BUILD_GEN_MDL
  else if (context.getShaderGenerator().getTarget() == mx::MdlShaderGenerator::TARGET)
  {
  mx::ShaderPtr shader = createShader(elem->getNamePath(), context, elem);
  @@ -1429,6 +1432,7 @@ void Viewer::saveShaderSource(mx::GenContext& context)
  writeTextFile(pixelShader, sourceFilename);
  new ng::MessageDialog(this, ng::MessageDialog::Type::Information, "Saved MDL source: ", sourceFilename);
  }
  +#endif
  }
  }
  catch (std::exception& e)
  