Since there were no more comments, I've installed (a tiny variant of) this
patch as 00ea749f2a.

Philipp Stephani <p.stephani2@gmail.com> schrieb am Fr., 21. Sep. 2018 um
15:28 Uhr:

> * Makefile.in (includedir): New variable.
> (install-arch-indep): Install emacs-module.h.
> (uninstall): Uninstall emacs-module.h.
> ---
>  Makefile.in | 6 ++++++
>  1 file changed, 6 insertions(+)
>
> diff --git a/Makefile.in b/Makefile.in
> index 19bf7c423f..5346429264 100644
> --- a/Makefile.in
> +++ b/Makefile.in
> @@ -151,6 +151,9 @@ libexecdir=
>  # Currently only used for the systemd service file.
>  libdir=@libdir@
>
> +# Where to install emacs-module.h.
> +includedir=@includedir@
> +
>  # Where to install Emacs's man pages.
>  # Note they contain cross-references that expect them to be in section 1.
>  mandir=@mandir@
> @@ -558,6 +561,8 @@ set_installuser=
>  ## See also these comments from 2004 about cp -r working fine:
>  ## https://lists.gnu.org/r/autoconf-patches/2004-11/msg00005.html
>  install-arch-indep: lisp install-info install-man
> ${INSTALL_ARCH_INDEP_EXTRA}
> +       $(MKDIR_P) -m 0755 $(includedir)
> +       $(INSTALL_DATA) src/emacs-module.h $(includedir)/emacs-module.h
>         -set ${COPYDESTS} ; \
>         unset CDPATH; \
>         $(set_installuser); \
> @@ -741,6 +746,7 @@ install-strip:
>  ###
>  ### Don't delete the lisp and etc directories if they're in the source
> tree.
>  uninstall: uninstall-$(NTDIR) uninstall-doc
> +       rm -f $(includedir)/emacs-module.h
>         $(MAKE) -C lib-src uninstall
>         -unset CDPATH; \
>         for dir in "$(DESTDIR)${lispdir}" "$(DESTDIR)${etcdir}" ; do    \
> --
> 2.19.0
>
>