deactivate clock_gettime on SunOS, as it does not work for threads.

deactivate clock_gettime on SunOS, as it does not work for threads.
CLOCK_THREAD_CPUTIME_ID is declared, but is unimplemented, execution fails and we don't check it.
This can be checked by the absence of _POSIX_THREAD_CPUTIME in unistd.h
Sadly, deactivating posix_gettime only for thread timers is not practical in xbt, so just default to another for all timers.