glibtoolize on MacOS tells me to consider adding `-I m4' to ACLOCAL_AMFLAGS, but that produces an error

It needs to be:

ACLOCAL_AMFLAGS = -I m4

in Makefile.am and:

AC_CONFIG_MACRO_DIR([m4])

in configure.ac. You do have an m4 directory, at $(top_srcdir) right?