Use system copy of qtsingleapplication.

--- musique.pro
+++ musique.pro
@@ -16,7 +16,7 @@
 TARGET = $${APP_UNIX_NAME}
 
 QT += network xml sql
-include(src/qtsingleapplication/qtsingleapplication.pri)
+CONFIG += qtsingleapplication
 
 HEADERS += src/mainwindow.h \
     src/aboutview.h \