Trying to set a customization variable that is not known as a valid
customization variable in texi2any is an error. It is possible, however,
to add new customization variables from init files. To add a customization
variable, the function is texinfo_add_valid_customization_option:
$variable_name is added as a valid customization variable name.
The variable value, if set, should also be available in the converters and therefore in the init file functions registered and called from the converters.