The makeinfo path now needs to be substituted in
libinterp/corefcn/help.h, not libinterp/corefn/help.cc. I have addressed
this in the attached patch.

I have also filed a bug upstream asking them to add a configure flag
--with-makeinfo. Hopefully, we won't have to manually substitute
the path to makeinfo in the next release.

https://savannah.gnu.org/bugs/index.php?55371