Our main processor, xsltproc, is not a general-purpose compiler, and does not “call” external programs. That is the primary purpose of the mbx script. You will see a configuration file, script/mbx.cfg, as part of the distribution. Read the comments at the top of this file, but foremost, realize that you are not meant to edit this file. It is a template, and any changes you make will be overwritten with an original version when you update. Instead, make a copy and place it as mathbook/user/mbx.cfg. The script will look for this copy first, then fallback to the generic version.