Increase CMake minimum version (needed for building famfs on a system with latest CMake)

When trying to build famfs (https://github.com/jagalactic/famfs) on an Arch linux system with a new CMake (cmake version 4.0.3-dirty), the build fails because building this repo has too old of a minimum CMake version. Bumping the minimum version here to v3.5 appears to make famfs build successfully (at least make clean all; echo $? prints 0...).

See the commit message for the exact error I got.

I am not that familiar with CMake so I don't have the experience to know whether it would be better to use the suggested <min>...<max> syntax to support a range of versions. If you feel that would be better, let me know. Or if you would prefer another way to resolve this build issue, let me know as well -- this is just the simplest thing that made famfs build successfully in my environment.

It does appear that CMake v3.5 is old enough by now that I expect most distributions someone would likely use today should provide a CMake newer than 3.5.