[autobuild] clock_gettime() -lrt with glibc < 2.17
clock_gettime() needs -lrt with glibc < 2.17,
and possibly other platforms
On systems without clock_gettime (-cough- Mac OSX -cough-),
use gettimeofday() (deprecated in POSIX.1-2008) which is slightly
lower precision, but reasonably fast in execution. References:
http://stackoverflow.com/questions/
5167269/clock-gettime-alternative-in-mac-os-x
http://stackoverflow.com/questions/
11680461/monotonic-clock-on-osx
https://discussions.apple.com/thread/
6023936?tstart=0