Does not build with OCaml 5

[hoheinzollern]

If it's unsupported yet then it should be documented, otherwise this looks like a simple fix.

I get the following error:

ocamlopt -c generate_index.mli
(for i in header footer css js redirect; do \
         echo "let $i = {xxx|"; \
         cat coq2html.$i; \
         echo '|xxx}'; \
         done) > resources.ml
ocamlopt -c resources.ml
ocamlopt -c generate_index.ml
File "_none_", line 1:
Alert ocaml_deprecated_auto_include: 
OCaml's lib directory layout changed in 5.0. The str subdirectory has been
automatically added to the search path, but you should add -I +str to the
command-line to silence this alert (e.g. by adding str to the list of
libraries in your dune file, or adding use_str to your _tags file for
ocamlbuild, or using -package str for ocamlfind).

File "generate_index.ml", line 167, characters 19-28:
167 |   String.(compare (lowercase s1) (lowercase s2))
                         ^^^^^^^^^
Error: Unbound value lowercase
make: *** [Makefile:10: generate_index.cmx] Error 2

[affeldt-aist]

Fixed by PR #38