* tta/C/texi2any.c (main): oad a Perl interpreter for the options
triggering calling Perl functions from C, DOCUMENTLANGUAGE_COLLATION
and COLLATION_LANGUAGE.  Handle txi_sort_element_counts return value
being NULL, even though it cannot happen right now because
txi_sort_element_counts aborts rather than returning NULL.
Add some const.

* tta/C/convert/texinfo.c (txi_sort_element_counts): abort if the
external_module argument is NULL.