(cgt_time_start): Likewise.
(cgt_time_end): Likewise.
(clock_time_start): Likewise.
(clock_time_end): Likewise.
(time_function): Read clock via function pointers time_start and
time_end, so we can select method at runtime.
(xalloc): Use die function.
(main): Choose timing function. If available, try clock_gettime,
and fall back to clock if it doesn't exist.

Rev: nettle/examples/nettle-benchmark.c:1.19