Throw out the conventional wisdom and base the decision on a micro
benchmark.

clock_gettime() is now preferred if it is consistently at least
double as fast as gethrtime(), which is the case on varnishdev-il,
the SmartOS vtest machine.

config.log gives details on the performance check, sample output
below:

configure:22703: ./conftest
hrtime              45989530 check 16748699083977959327
clock_gettime        4119385 check 16748701613138517215
...
hrtime              48113108 check 16748749015170035860
clock_gettime        4020802 check 16748751585081458308
clock_gettime wins 10/10