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.