Support old QT5.12.

src/SeerAboutDialog.cpp

@@ -23,7 +23,11 @@ SeerAboutDialog::SeerAboutDialog (QWidget* parent) : QDialog(parent) {
     text.replace("VERSIONNUMBER", Seer::version());
 
     // Put the About text in as markdown. Move back to the begining.
+#if (QT_VERSION >= QT_VERSION_CHECK(5, 14, 0))
     textBrowser->setMarkdown(text);
+#else
+    textBrowser->setText(text);
+#endif
     textBrowser->moveCursor (QTextCursor::Start);
 
     // Set the TextBrowser's background to the same as the window's.