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.cpp

@@ -6,6 +6,7 @@
 
 #include <QtCore/QString>
 #include <QtCore/QStringList>
+#include <cstdarg>
 #include "qt_str.h"
 
 namespace nethack_qt_ {
@@ -80,4 +81,20 @@ int cp437(int ch)
     return cp437table[(unsigned char)ch];
 }
 
+QString
+nh_qsprintf(const char *format, ...)
+{
+    QString msg;
+    std::va_list args;
+
+    va_start(args, format);
+#if QT_VERSION >= QT_VERSION_CHECK(5, 5, 0)
+    msg = QString::vasprintf(format, args);
+#else
+    msg.vsprintf(format, args);
+#endif
+    va_end(args);
+    return msg;
+}
+
 } // namespace nethack_qt_