From https://stackoverflow.com/a/17358491/50552 I learned that cmake caches environment variables in a file in the build directory. 
So you have to “rm -rf release/*” before retrying.
That helped, when called with --check-system-vars, cmake finds boost:
[me@machine release]$ cmake --check-system-vars ..
-- Found Boost: /usr/include (found suitable version "1.66.0", minimum required is "1.66.0") found components: coroutine context date_time filesystem iostreams thread system program_options regex chrono atomic