Implement libtoolize suggestion setting ACLOCAL_AMFLAGS

Running ./autogen.sh reports this suggestion:
    libtoolize: Consider adding `-I m4' to ACLOCAL_AMFLAGS in Makefile.am.

Add suggested setting as it causes no difference and silences the
suggestion.

The Libtool Manual, 5.5.1 Invoking libtoolize
https://www.gnu.org/software/libtool/manual/html_node/Invoking-libtoolize.html

    "If libtoolize detects an explicit call to AC_CONFIG_MACRO_DIRS (see
    The Autoconf Manual in The Autoconf Manual) in your configure.ac, it
    will put the Libtool macros in the specified directory.

    In the future other Autotools will automatically check the contents
    of AC_CONFIG_MACRO_DIRS, but at the moment it is more portable to
    add the macro directory to ACLOCAL_AMFLAGS in Makefile.am, which is
    where the tools currently look. If libtoolize doesn't see
    AC_CONFIG_MACRO_DIRS, it too will honour the first '-I' argument in
    ACLOCAL_AMFLAGS when choosing a directory to store libtool
    configuration m...