build: Silence libtoolize warnings

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>

Makefile.am

@@ -1,3 +1,5 @@
+ACLOCAL_AMFLAGS = -I m4
+
 bin_PROGRAMS =
 noinst_PROGRAMS =
 libexec_PROGRAMS =

configure.ac

@@ -18,6 +18,7 @@ AC_SUBST([WESTON_VERSION], [weston_version])
 
 AC_CONFIG_AUX_DIR([build-aux])
 AC_CONFIG_HEADERS([config.h])
+AC_CONFIG_MACRO_DIR([m4])
 
 AC_USE_SYSTEM_EXTENSIONS
 AC_SYS_LARGEFILE