fixed linking Realtime Extensions library on some linux systems (undefin...

...ed reference to clock_gettime)

This should fix the problem discussed in: https://github.com/KhronosGroup/glTF/issues/211
It just links 'librt' explicitly when on a linux system.

I had the error discussed in the above thread occurring on Ubuntu 12.04.4 LTS and CentOS 6.5.

Please review.

Toni