[boost] [chrono] mac thread_clock support

I noticed that chrono doesn't seem to support thread_clock on mac (the posix implementation doesn't work for it) so I wrote up an implementation. https://github.com/boostorg/chrono/pull/1 I was pretty surprised this didn't already exist, but haven't been able to find anything in the mailing list or trac regarding it. Curious to hear if there was a reason to avoid this implementation or if it's just something no one got around to. Posting here for visibility since the git / Github workflow is relatively new to a lot of people here.