Our main processor, xsltproc, is not a general-purpose compiler, and does not “call” external programs. That is the raison d'être of the pretext/pretext script. You will see a configuration file, pretext/pretext.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 user/pretext.cfg within the distribution. For instance, if your distribution is in the mathbook directory, then the commands
cd /path/to/mathbook

would be appropriate. The script will look for the user copy first, and if not found, then fallback to the generic version.