Add support for Ubuntu 17.04 and 17.10.

Basically, build_helper has been adapted so that:
  ./buil_oai -I -w USRP
works for Ubuntu 17.04 and 17.10.

Concerning those systems:
- compilation with "--eNB --UE -w USRP" works
- the enb softmodem runs on 17.04 (not tested on 17.10)
- nothing else has been tested.

Users should report any problem.