Yes, Jens, you're right.
I forgot to mention that LT_INIT is a non-backward compatible feature of libtool 2.2.6, which is not there in prior versions (which use AC_PROG_LIBTOOL).
Could you do your test again, with the same older platform, yet after replacing LT_INIT and LT_PREREQ with plain AC_PROG_LIBTOOL and AC_DISABLE_STATIC in configure.in, leaving other patched changes as is?
I'd be curious to know the results. On my platform, leaving these is OK after all, yet you still have to add AC_CONFIG_MACRO_DIR([m4]) to configure.in with the patch.
Fab