bug#23870: Guile 2.0.11 can't find clock_getcpuclockid on macOS 10.12 Sierra

[Rahul AG]

Guile version: 2.0.11
Machine type: x86_64-apple-darwin16.0.0

macOS 10.12 introduces the `clock_gettime` family of functions, with slight 
differences from the *BSD and Linux implementations.

The failure in linking is caused by a call at libguile/stime.c @ L841 to 
`clock_getcpuclockid`, which does NOT exist on macOS, while `_POSIX_CPUTIME` 
and `CLOCK_PROCESS_CPUTIME_ID` do exist.

I have an absolutely hideous patch[1] locally applied that gets things 
compiling (and get-internal-run-time seems to work), but is otherwise terrible 
from a readability and maintainability standpoint. That said, narrowing it down 
to that single call should help make this fix easier.

Cheers.
Rahul

[1] https://gist.github.com/rahulg/baa500e84136f0965e9ade2fb36b90ba