You're building against Qt 4, but this code apparently is Qt 5 only and relies on signals being public in Qt 5 (they were protected in Qt 4).
Look at the include paths set for g++:
g++ -c [...] -I/usr/include/qt4/QtCore -I/usr/include/qt4/QtNetwork -I/usr/include/qt4 -I. -o qftp.o qftp.cpp
The Qt 4 includes of your system-wide installation are used.
I assume that's an accident and you just have to choose the correct Qt version (Qt 5.1.1) in Qt Creator.