message: qt: Use "fusion" style on macOS Big Sur with old Qt
  The "macintosh" style is broken on macOS Big Sur at least for Qt 5.9.8.
diff: #include <QThread>
  #include <QTimer>
  #include <QTranslator>
  +#include <QtGlobal>
  #if defined(QT_STATICPLUGIN)
  #include <QtPlugin>
  @@ -466,6 +467,13 @@ int GuiMain(int argc, char* argv[])
  QCoreApplication::setAttribute(Qt::AA_EnableHighDpiScaling);
  #endif
  +#if (QT_VERSION <= QT_VERSION_CHECK(5, 9, 8)) && defined(Q_OS_MACOS)
  + const auto os_name = QSysInfo::prettyProductName();
  + if (os_name.startsWith("macOS 11") || os_name.startsWith("macOS 10.16")) {
  + QApplication::setStyle("fusion");
  + }
  +#endif
  +
  BitcoinApplication app;
  /// 2. Parse command-line options. We do this after qt in order to show an error if there are problems parsing these
  