That's weird. I'm getting a similar error but cmake did generate all the project files without any problem.
Code:
-- Found Nasm 2.15.05 to build assembly primitives
-- Could NOT find Ruby (missing: Ruby_LIBRARY) (found suitable version "2.7.1", minimum required is "1.8.0")
-- x265 version 3.4+13-g729a838d3+38