Fixes cmake uavcan external project for cmake 2.8.12.
Fixes cmake uavcan external project for cmake 2.8.12.

jlecoeur/Firmware,PX4/Firmware,darknight-007/Firmware,krbeverx/Firmware,darknight-007/Firmware,mcgill-robotics/Firmware,jlecoeur/Firmware,PX4/Firmware,jlecoeur/Firmware,acfloria/Firmware,dagar/Firmware,Aerotenna/Firmware,Aerotenna/Firmware,krbeverx/Firmware,mcgill-robotics/Firmware,Aerotenna/Firmware,krbeverx/Firmware,mcgill-robotics/Firmware,dagar/Firmware,darknight-007/Firmware,krbeverx/Firmware,Aerotenna/Firmware,acfloria/Firmware,acfloria/Firmware,PX4/Firmware,dagar/Firmware,jlecoeur/Firmware,Aerotenna/Firmware,Aerotenna/Firmware,jlecoeur/Firmware,krbeverx/Firmware,mcgill-robotics/Firmware,mje-nz/PX4-Firmware,darknight-007/Firmware,mje-nz/PX4-Firmware,darknight-007/Firmware,mcgill-robotics/Firmware,dagar/Firmware,mcgill-robotics/Firmware,mje-nz/PX4-Firmware,dagar/Firmware,acfloria/Firmware,jlecoeur/Firmware,mje-nz/PX4-Firmware,PX4/Firmware,dagar/Firmware,mje-nz/PX4-Firmware,krbeverx/Firmware,acfloria/Firmware,acfloria/Firmware,PX4/Firmware,jlecoeur/Firmware,Aerotenna/Firmware,mje-nz/PX4-Firmware,dagar/Firmware,PX4/Firmware,krbeverx/Firmware,acfloria/Firmware,mje-nz/PX4-Firmware,PX4/Firmware,jlecoeur/Firmware,mcgill-robotics/Firmware