The install-strip target hasn't been right for a long time.
Library stripping was even removed in checkin [517d7010fc] due to problems.

The way to do it is to set the STRIPPROG env var,
similar to how the CPPROG env var is set for DIST_INSTALL_*.

This way, install-sh can simply be updated from
[https://git.savannah.gnu.org/cgit/automake.git/tree/lib/install-sh]
without having to fiddle with it, which is wrong anyway,
since passing strip options to any install probably won't work ... anywhere?

Latest version of install-sh is 2020-07-26.22.

I think this will still need to be tested to vis-a-vis [517d7010fc]
to see what effect it has on that problem.

The Makefile.in patch would look like this:

-INSTALL_PROGRAM="$(INSTALL_PROGRAM) ${INSTALL_STRIP_PROGRAM}" \
-INSTALL_LIBRARY="$(INSTALL_LIBRARY) ${INSTALL_STRIP_LIBRARY}"
+INSTALL_PROGRAM="STRIPPROG='strip ${INSTALL_STRIP_PROGRAM}' $(INSTALL_PROGRAM) -s" \
+INSTALL_LIBRARY="STRIPPROG='strip ${INSTALL_STRIP_LIBRARY}' $(INSTALL_LIBRARY) -s"