util/u_thread: Check for pthread_getcpuclockid.

pthread_getcpuclockid is not available on macOS.

Suggested-by: Eric Engestrom <eric.engestrom@intel.com>
Fixes: 4913215d ("util/u_thread: don't restrict u_thread_get_time_nano() to __linux__")
Signed-off-by: Vinson Lee <vlee@freedesktop.org>
42 jobs for !2995 with fix-no-pthread_getcpuclockid in 17 minutes and 59 seconds (queued for 8 seconds)
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