build errors on Fedora 24 and CentOS 7

[Brian Warner]

On 10/12/16 9:33 AM, Lukas Pirl wrote:

> Well, after a few minutes stepping around, pdb told me the reason for
> this failure. It was quite trivial in the end: the path to `ip` is just
> different on RHEL-based systems.

Aha! I'd bet that previous RHEL systems still had ifconfig somewhere, so
it didn't matter that the code was looking for 'ip' in the wrong place.

PR merged. Thanks!
 -Brian