For other people’s reference, I solved the issue by editing my platform mkspec file:
/users/nic/Qt/5.3/clang_64/mkspecs/qdevice.pri
change the following line:
!host_build:QMAKE_MAC_SDK = macosx10.8
to this:
!host_build:QMAKE_MAC_SDK = macosx10.9
Be sure to clean your project and run qmake again.
Nic.