build: apply autoconf suggestions

add -I m4 to Makefile for ACLOCAL_AMFLAGS
add macro dirs to configure_ac

Change-Id: Ied5ca518fedab65c63d0c89bcd0f21c3d32265e1
Signed-off-by: Patrick Venture <venture@google.com>
diff --git a/configure.ac b/configure.ac
index db3bc83..325cc0e 100644
--- a/configure.ac
+++ b/configure.ac
@@ -3,6 +3,7 @@
 AC_INIT([phosphor-ipmi-blobs], [1.0], [https://github.com/openbmc/phosphor-ipmi-blobs/issues])
 AC_LANG([C++])
 AC_CONFIG_HEADERS([config.h])
+AC_CONFIG_MACRO_DIRS([m4])
 AM_INIT_AUTOMAKE([subdir-objects -Wall -Werror foreign dist-xz])
 AM_SILENT_RULES([yes])