Merge bitcoin-core/gui#177: Use "fusion" style on macOS Big Sur with old Qt

4e1154dfd1 qt: Use "fusion" style on macOS Big Sur with old Qt (Hennadii Stepanov)

Pull request description:

  The "macintosh" style is broken on macOS Big Sur:

  - https://github.com/bitcoin/bitcoin/issues/20555#issuecomment-756264648
  - #136

ACKs for top commit:
  MarcoFalke:
    review ACK 4e1154dfd1 can't test
  jarolrod:
    ACK 4e1154dfd1
  jonasschnelli:
    Tested ACK 4e1154dfd1

Tree-SHA512: c2e0f7be220c8b34b182c73e362f41d0e8c8c002e766fcb5491c62f3cfb9f70eabbd32b29baefa152135efc5f83b15534c1c2459e500a586b0f64c5aa8acf614

src/qt/bitcoin.cpp

@@ -51,6 +51,7 @@
 #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