Drop support for OS X 10.11. Apple screwed this pooch too many different ways.

Last straw was the ahipped headers in some versions not matching what's on
the Apple website (Report from Leonardo Bruni Schenkel.)

We also have intelligence that the clock_setting shim in some builds
of 10.11 is broken.

10.12 has clock_gettime(2) and clock_settime(2); it should be OK.