I don't think there is a simple way of supporting the global menu of Ubuntu 13 Unity, short of compiling a separate, special package for that specific platform / version.
It could be related to QCAD being compiled on Ubuntu 10 to provide backwards compatibility with older Linux installations. Ubuntu 10 does not come with a global menu.
Aside from this, it is possible to use the installed Qt version instead of the Qt version that comes with QCAD (simply remove the Qt libraries from the QCAD installation directory). However, that does not seem to have any influence on the menu location.
What seems to be possible is to go the other way: disable the global menu for all applications:http://lifehacker.com/5887462/how-to-disable-ubuntus-annoying-global-menu-bar