Fix a crash (#4037)

This code loads a Qt5 library, which will cause problems if done from a Qt4 application. If the application is Qt4 based we don't have a bug with kde changing the menus anyway, so we can skip this code.

src/gui/MainWindow.cpp

@@ -65,7 +65,7 @@
 
 #include "lmmsversion.h"
 
-#if !defined(LMMS_BUILD_WIN32) && !defined(LMMS_BULID_APPLE) && !defined(LMMS_BUILD_HAIKU)
+#if !defined(LMMS_BUILD_WIN32) && !defined(LMMS_BULID_APPLE) && !defined(LMMS_BUILD_HAIKU) && QT_VERSION >= 0x050000
 //Work around an issue on KDE5 as per https://bugs.kde.org/show_bug.cgi?id=337491#c21
 void disableAutoKeyAccelerators(QWidget* mainWindow)
 {
@@ -92,7 +92,7 @@ MainWindow::MainWindow() :
 	m_metronomeToggle( 0 ),
 	m_session( Normal )
 {
-#if !defined(LMMS_BUILD_WIN32) && !defined(LMMS_BULID_APPLE) && !defined(LMMS_BUILD_HAIKU)
+#if !defined(LMMS_BUILD_WIN32) && !defined(LMMS_BULID_APPLE) && !defined(LMMS_BUILD_HAIKU) && QT_VERSION >= 0x050000
 	disableAutoKeyAccelerators(this);
 #endif
 	setAttribute( Qt::WA_DeleteOnClose );