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

@@ -29,12 +29,15 @@
 
 #include "programupdater.h"
 
+#include <QtCore/qconfig.h>
 #include <QtSystemDetection>
 #include <QDebug>
 #include <QDesktopServices>
 #include <QRegularExpression>
 #include <QXmlStreamReader>
 
+#include <libtorrent/version.hpp>
+
 #include "base/global.h"
 #include "base/net/downloadmanager.h"
 #include "base/preferences.h"
@@ -98,11 +101,41 @@ void ProgramUpdater::rssDownloadFinished(const Net::DownloadResult &result)
     };
 
 #ifdef Q_OS_MACOS
-    const QString OS_TYPE = u"Mac OS X"_s;
+#define BASE_OS "Mac OS X"
 #elif defined(Q_OS_WIN)
-    const QString OS_TYPE = u"Windows x64"_s;
+#define BASE_OS "Windows x64"
+#endif
+
+#define CONCAT_(a, b) a ## b
+#define CONCAT(a, b) CONCAT_(a, b)
+#define STRINGIFY(s) #s
+
+#if (QT_VERSION_MAJOR == 6) && (LIBTORRENT_VERSION_MAJOR == 1)
+#define _TYPE() BASE_OS
+#define TYPE CONCAT(u, _TYPE())
+
+#elif (QT_VERSION_MAJOR == 6) && (LIBTORRENT_VERSION_MAJOR > 1)
+#define _TYPE(lt_major, lt_minor) BASE_OS " (lt" STRINGIFY(lt_major) STRINGIFY(lt_minor) ")"
+#define TYPE CONCAT(u, _TYPE(LIBTORRENT_VERSION_MAJOR, LIBTORRENT_VERSION_MINOR))
+
+#elif (QT_VERSION_MAJOR > 6)
+#define _TYPE(qt_major, lt_major, lt_minor) BASE_OS " (qt" STRINGIFY(qt_major) " lt" STRINGIFY(lt_major) STRINGIFY(lt_minor) ")"
+#define TYPE CONCAT(u, _TYPE(QT_VERSION_MAJOR, LIBTORRENT_VERSION_MAJOR, LIBTORRENT_VERSION_MINOR))
+
+#else
+#error "OS_TYPE is undefined for the current Qt/libtorrent combination"
 #endif
 
+    const QString OS_TYPE = TYPE""_s;
+
+#undef TYPE
+#undef _TYPE
+#undef STRINGIFY
+#undef CONCAT
+#undef CONCAT_
+#undef BASE_OS
+
+
     bool inItem = false;
     QString version;
     QString updateLink;