Issues with clock_gettime(CLOCK_REALTIME, &wait) pre macOS 10.13

[Chris Jones]

Hi,

> I am interested in working on this, as I think it would ease some issues 
> in a few ports I have stumbled over recently. Let me try and put 
> something to test together.

I have started to put something together for this. I have a little new 
project in github

https://github.com/cjones051073/macports-legacy-support

and then a new port that installs the headers from this repo.

<https://github.com/cjones051073/macports-ports/commit/f1b25a6dea9e895ae6d1d14411099b6cd43153ea>

I have run a first quick test, building xrootd, and the signs are 
positive in that as I hoped it fixed the issues I was seeing with xrootd 
completely transparently, without having to patch the port or code in 
anyway. Obviously much more testing is needed though, so please if you 
are interested (Ken?) checkout my ports branch above and give it a try 
yourself.

Also, if you have any suggestions for additional header fixes to add, 
let me know (or make a PR against the project...). ( Longer term, if 
this idea has legs, I think the git repo should move from my account to 
the main MacPorts area, but this can wait. )

Chris