Linux compile workaround

[palerikm]

See cbor issue: cabo/cn-cbor#22

Do not build the cbor tests when doing the external build and have optimalization turned on?

OS: Linux ubuntu 3.19.0-16-generic #16-Ubuntu SMP Thu Apr 30 16:09:58 UTC 2015 x86_64 x86_64 x86_64 GNU/Linux
CC: cc (Ubuntu 4.9.2-10ubuntu13) 4.9.2