pingus-0.7.6 fails to build with clang

[pacho2]

It can be reproduced on Gentoo:
https://bugs.gentoo.org/739312

And Debian:
https://clang.debian.net/status.php?version=13.0.0&key=NO_CAT
https://clang.debian.net/logs/2021-10-25-13.0.0/pingus_0.7.6-5.1_unstable_clang13.0.0.log

Build fails with this error:

In file included from src/lisp/getters.cpp:2:
src/lisp/getters.hpp:33:15: error: no viable conversion from 'std::shared_ptr<Lisp>' to 'const lisp::Lisp *'
  const Lisp* el = lisp->get_list_elem(1);
              ^    ~~~~~~~~~~~~~~~~~~~~~~
1 error generated.
scons: *** [build/src/lisp/getters.o] Error 1
scons: building terminated because of errors.
make: *** [debian/rules:24: build-stamp] Error 2

Thanks for your help