When running the autogen.sh script, libtoolize complains thusly:

	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.

Silence the warnings by following libtoolize's advice.

Signed-off-by: Thierry Reding <treding@nvidia.com>