To make simos first set the following environment variables:

        SIMOS_DIR - the installation directory where the "build_files"
                    child directory resides. This is the location of 
                    several libraries and include files.
        CPU       - This is the target platform that the simos binary
                    will be built for. The current choices are:
                        SGI,SGIN32,SGIM4,SUN,ALPHA,X86

Then type gmake in the src directory (the directory you are in right now).
Note that gmake is called "gnumake" on some systems.