I had errors below when running cmake, and installing libssh through MacPorts fixed them. Perhaps libssh be added to the list of required dependancies on https://www.ovito.org/manual/development.build_macosx.html.
-- Could NOT find LIBSSH (missing: LIBSSH_LIBRARY LIBSSH_INCLUDE_DIR)
CMake Error: The following variables are used in this project, but they are set to NOTFOUND.
Please set them or make sure they are set and tested correctly in the CMake files:
used as include directory in directory /Users/Menachem/Research/ovito/src/core
linked by target "Core" in directory /Users/Menachem/Research/ovito/src/core