Minor and low prio, but:

qtsingleapplication/qtsingleapplication.h and other headers in qtsingleapplication/ miss standard "#ifndef" statements so including (also indirectly; we run into this when trying to start using precompiled headers) the header twice obviously leads to

qtsingleapplication/qtsingleapplication.h:68: error: redefinition of 'class QtSingleApplication'
qtsingleapplication/qtsingleapplication.h:69: error: previous definition of 'class QtSingleApplication'

(note that headers in e.g. qtsoap solution do have correct

#ifndef QTSOAP_H
#define QTSOAP_H
...
#endif

=> I didn't check other solutions)