@sw2wolf:
I'd need more info about your environment. For example, what gcc version you use, etc. There are ways to override that in the respective nimrod.cfg in the github source, like this:
gcc.exe = "gcc46" gcc.linkerexe = "gcc46"
Actually, nimrod compiles just fine with the shipped gcc 4.2 of FreeBSD 9. Your error there indicates problems with your build environment. Either that, or the sysctl API changed in that specific version of FreeBSD.
Paste your sysctl.h somewhere, I will compare that to mine.
today, i download nimrod_0.9.0.zip and recompile. It is strange. All works great.
thanks!