When you tell VirtualBox to enable USB support in a virtual machine, VirtualBox says it cannot comply until you install the Oracle VM VirtualBox Extension Pack. According to https://www.virtualbox.org/wiki/Downloads , that extension pack is under the VirtualBox Personal Use and Evaluation License (PUEL), and reading over that licence at https://www.virtualbox.org/wiki/VirtualBox_PUEL , it forbids modifications (section 2 and 3). It looks like a licence holder may not be allowed to redistribute even unmodified copies either, though I'm unsure of that part.
FYI: Virtualbox in Ubuntu quantal requires a non-free compiler.
https://trisquel.info/en/forum/virtualbox#comment-39916
Ruben, please add the following to helpers/make-virtualbox to build without support for USB 2.0: