The user-customizable variables and functions are located in the package “CUSTOM” and thus can be listed using (APROPOS "" "CUSTOM"):

You should set these variables (and do whatever other customization you see fit) in the file config.lisp in the build directory before building CLISP. Alternatively, after building CLISP, or if you are using a binary distribution of CLISP, you can modify config.lisp, compile and load it, and then save the memory image. Finally, you can create an RC file which is loaded whenever CLISP is started.