https://dev.mysql.com/doc/refman/8.0/en/installing-source-distribution.html
cmake .. -G "Visual Studio 14 2015 Win64" -DDOWNLOAD_BOOST=1 -DWITH_BOOST=boostdir -DOPENSSL_ROOT_DIR=OpenSSL-Win64 -DOPENSSL_LIBRARIES=OpenSSL-Win64/lib
https://www.cloudinsidr.com/content/how-to-install-the-most-recent-version-of-openssl-on-windows-10-in-64-bit/
https://slproweb.com/products/Win32OpenSSL.html
It is a common misunderstanding: CMake does not need environment variables to know where the library and include dir are but CMake variables.
Since CMake can not find your OpenSSL lib and include directory, you will have to manually tell him where they are with its command line when you call it. Use the option -D to set constants in CMake from the command line. You will need to set the constants OPENSSL_ROOT_DIR and OPENSSL_LIBRARIES since they are the one triggering the error.
cmake -DOPENSSL_ROOT_DIR=/usr/local/ssl -DOPENSSL_LIBRARIES=/usr/local/ssl/lib