A bit more information would be helpful (what platform, what version of OMNET, what version of INET).
But my guess is, you are trying to build it on Windows, with OMNeT 4.5.
There is a known problem with the make command bundled in OMNET 4.5 when you are trying to do a parallel build (which is turne don by default for INET).
Solutions:
- Use OMNET 4.6RC
- Use Linux or Mac
- Turn off the parallel build in the IDE (project properties / C++ Build)
- build INET from the command line
$ make makefiles
$ make