[Simh] What library is "-lrt" in makefile for?

[Galen Tackett]

I'm trying to build V3.7-0030 on my OS X V10.3.9, but make errors out 
telling me "can't find file for -lrt". I'm not a Unix programmer so I 
hope someone can tell me what the "rt" library is and whether it should 
be on my OS X system.

Thanks,

Galen