fix cmake error/warning about empty file
fix cmake error/warning about empty file

hpi-xnor/BMXNet,hpi-xnor/BMXNet,hpi-xnor/BMXNet,hpi-xnor/BMXNet,hpi-xnor/BMXNet,hpi-xnor/BMXNet,hpi-xnor/BMXNet,hpi-xnor/BMXNet,hpi-xnor/BMXNet