On make install Rhash 1.3.9 installs /usr/local/lib/librhash.so.0, but does not install /usr/local/lib/librhash.so.

Then cmake cannot find librhash.so on the system, see https://gitlab.kitware.com/cmake/cmake/-/issues/20728#note_762345 and the comments that follow.

In particular, there are some rules followed by ldconfig, which I do not remember exactly, how to generate the symlinks, based on SONAME and such things. Creating manually the link librhash.so → librhash.so does not help cmake to find librhash. Also, after the symlink is created manually and ldconfig is run, ldconfig -p |grep rhash prints only the the file with .so.0 at the end.