Make Program Updater choose the same build for download

We're probably stuck offering the duo of RC_1_2 and RC_2_0 for some
time in the future. So hardcode the choices and make the Program Updater
choose the variant the user currently uses.

src/gui/programupdater.cpp

@@ -29,6 +29,9 @@
 
 #include "programupdater.h"
 
+#include <libtorrent/version.hpp>
+
+#include <QtCore/qconfig.h>
 #include <QtSystemDetection>
 #include <QDebug>
 #include <QDesktopServices>
@@ -61,6 +64,22 @@ namespace
         }
         return (newVersion > currentVersion);
     }
+
+    QString buildVariant()
+    {
+#if defined(Q_OS_MACOS)
+        const auto BASE_OS = u"Mac OS X"_s;
+#elif defined(Q_OS_WIN)
+        const auto BASE_OS = u"Windows x64"_s;
+#endif
+
+        if constexpr ((QT_VERSION_MAJOR == 6) && (LIBTORRENT_VERSION_MAJOR == 1))
+            return BASE_OS;
+        else
+            return u"%1 (qt%2 lt%3%4)"_s.arg(BASE_OS, QString::number(QT_VERSION_MAJOR), QString::number(LIBTORRENT_VERSION_MAJOR), QString::number(LIBTORRENT_VERSION_MINOR));
+
+        return {};
+    }
 }
 
 void ProgramUpdater::checkForUpdates() const
@@ -97,12 +116,7 @@ void ProgramUpdater::rssDownloadFinished(const Net::DownloadResult &result)
             : QString {};
     };
 
-#ifdef Q_OS_MACOS
-    const QString OS_TYPE = u"Mac OS X"_s;
-#elif defined(Q_OS_WIN)
-    const QString OS_TYPE = u"Windows x64"_s;
-#endif
-
+    const QString variant = buildVariant();
     bool inItem = false;
     QString version;
     QString updateLink;
@@ -128,7 +142,7 @@ void ProgramUpdater::rssDownloadFinished(const Net::DownloadResult &result)
         {
             if (inItem && (xml.name() == u"item"))
             {
-                if (type.compare(OS_TYPE, Qt::CaseInsensitive) == 0)
+                if (type.compare(variant, Qt::CaseInsensitive) == 0)
                 {
                     qDebug("The last update available is %s", qUtf8Printable(version));
                     if (!version.isEmpty())