build: apply autoconf suggestions

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

Change-Id: I759e777219457b32399bc6a3ae559f62c731c920
Signed-off-by: Patrick Venture <venture@google.com>
diff --git a/Makefile.am b/Makefile.am
index af68e35..d2dc190 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -1,3 +1,4 @@
+ACLOCAL_AMFLAGS = -I m4
 AM_DEFAULT_SOURCE_EXT = .cpp
 
 libfirmwareblobdir = ${libdir}/ipmid-providers