mtr 0.92 -> 0.93 breaks darwin build

[jwiegley]

The commit 32526e6 by @peterhoeg introduces a dependency on a Linux-only library, libcap. The previous version was working fine on darwin, so at the very least should be offered if 0.93 no longer functions there.

[7c6f434c]

Does overriding with libcap=null let you build 0.93?

[peterhoeg]

Yeah, this isn't the first time I break stuff on Darwin - apologies.

[jwiegley]

Thanks, @peterhoeg! :)