Restore compatibility with Qt 4

The test system is Slackware 14.2, which uses Qt 4.8.7.

* WANT_WIN_QT4 is defined, and has the expected meaning. Qt 5 is still
  the default.

* The QT_NO_SOUND macro now excludes all headers and declarations
  relating to sound; the multimedia package is not needed to build
  (on any Qt 4, 5 or 6).

* A new function, nh_qsprintf, replaces QString::asprintf, for Qt
  older than 5.5. These versions do not have QString::asprintf.

* DYNAMIC_STATUSLINES is disabled for Qt older than 5.9. These versions
  do not have QSplitter::replaceWidget.

win/Qt/qt_str.h

@@ -19,6 +19,13 @@ extern QString nh_capitalize_words(const QString& str);
 // Map symbol conversion
 extern int cp437(int ch);
 
+// sprintf
+extern QString nh_qsprintf(const char *format, ...)
+#if defined(__GNUC__) && (__GNUC__ >= 2)
+    __attribute__((format(printf, 1, 2)))
+#endif
+;
+
 } // namespace nethack_qt_
 
 #endif