Try to suppress the use of clock_gettime on Darwin which apparantly
defines _POSIX_CPUTIME but doesn't support the clock_* functions.

I don't test the value of _POSIX_CPUTIME because the spec merely says
that if it is defined, the CPU-specific timers are available, whereas it
says that _POSIX_TIMERS must be defined and defined to a value greater
than zero. However, this may not work, as the POSIX spec clearly states:

  "If the symbolic constant _POSIX_CPUTIME is defined, then the symbolic
  constant _POSIX_TIMERS shall also be defined by the implementation to
  have the value 200112L."

If this doesn't work, I'll add more hacks for Darwin.