[osx] We cannot use std::optional::value() if we want to support macOS 10.9

It looks like value() member was introduced in macOS 10.13 (error
detected because we use CMAKE_OSX_DEPLOYMENT_TARGET=10.9).

docs/CODING_STYLE.md

@@ -143,6 +143,8 @@ We are using C++17 standard. You can safely use:
 * Use `std::shared_ptr`, `std::unique_ptr`, or `base::Ref`
 * Use `std::clamp`
 * Use `std::optional`
+  * Use `std::optional::operator*` (because `std::optional::value` isn't
+    available on macOS 10.9, only since 10.13)
 * Use `static constexpr T v = ...;`
 * You can use `<atomic>`, `<thread>`, `<mutex>`, and `<condition_variable>`
 * Prefer `using T = ...;` instead of `typedef ... T`

src/app/app.cpp

@@ -636,7 +636,7 @@ bool App::isPortable()
                       base::get_file_path(base::get_app_path()),
                       "aseprite.ini"));
   }
-  return is_portable.value();
+  return *is_portable;
 }
 
 tools::ToolBox* App::toolBox() const