diff3(3) to generate the context diff files between revisions. The diff utilities suite needs to be installed on your system, and when you install RCS, the software will check for its presence.
Precompiled diffutils binaries are available at:
and its mirror sites. If you need to compile
diff(1), et al., from source, it is located at:
and its mirror sites.
You will also need to have the ELF libraries installed on your system if you want to install pre-built binaries. See the ELF-HOWTO for further details.