Link against librt.

Tools/CMake/torque3d.cmake

@@ -506,7 +506,7 @@ endif()
 
 if(UNIX)
     # copy pasted from T3D build system, some might not be needed
-	set(TORQUE_EXTERNAL_LIBS "dl Xxf86vm Xext X11 Xft stdc++ pthread GL" CACHE STRING "external libs to link against")
+	set(TORQUE_EXTERNAL_LIBS "rt dl Xxf86vm Xext X11 Xft stdc++ pthread GL" CACHE STRING "external libs to link against")
 	mark_as_advanced(TORQUE_EXTERNAL_LIBS)
     
     string(REPLACE " " ";" TORQUE_EXTERNAL_LIBS_LIST ${TORQUE_EXTERNAL_LIBS})