build: add -I m4 to Makefile for ACLOCAL_AMFLAGS

Per suggestion during configuration, add m4 directory to
ACLOCAL_AMFLAGS.

Change-Id: I0c7ae12e02e5ef25f41c8a3b31b5338504ff9463
Signed-off-by: Patrick Venture <venture@google.com>
1 file changed