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To install a ancient ffi (<= 1.9.18) in Catalina MacOS or newer OS we need to use this command: gem install ffi -v '1.9.18' -- --with-cflags="-Wno-error=implicit-function-declaration"

In a project I had troubles triying to install mysql2 gem with a older version of mariadb installed via homebrew. the solution is to send the path of the installation. gem install mysql2 -- --with-mysql-config=$(brew --prefix mariadb@10.2)/bin/mysql_config