I found some information from one project named BGP++( http://www.ece.gatech.edu/research/labs/MANIACS/BGP++/). In this project, NS-2 gets checkpointing support by Condor, and the compiling method is: "appending the -DHAVE_CONDOR on the DEFINE macro and the -lcondorsyscall in the LIB macro in the Makefile.in of NS-2". I tried this way, but failed. The error information is "/usr/bin/ld: cannot find -lcondorsyscall".