Add AC_CONFIG_MACRO_DIR([m4]), re-add ACLOCAL_AMFLAGS

Fixes warning:
libtoolize: putting auxiliary files in AC_CONFIG_AUX_DIR, `config'.
libtoolize: copying file `config/ltmain.sh'
libtoolize: Consider adding `AC_CONFIG_MACRO_DIR([m4])' to configure.ac and
libtoolize: rerunning libtoolize, to keep the correct libtool macros in-tree.
libtoolize: Consider adding `-I m4' to ACLOCAL_AMFLAGS in Makefile.am.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>