Fix for build failure on Wheezy and possibly other distros.

Hi Rusty, this almost compiled on my netbook but not quite, somewhere along the way something changed in various distros with the defaults used for librt

Fix build fail on missing clock_gettime() on Wheezy (at least) 7.2 32-bit, and probably openSuse 12.2 and others.

Note: this is not using configure/automake or cmake or similar so I am unsure of the 'best' way to take into account distro idiosyncracies in this case. Tested on Wheezy 32-bit, and I know from recent experience after upgrading 11.4 --> 12.2 that -lrt is needed on Opensuse (for at least 64bit) as well.