configure.ac: fix autogen.sh

Put -I m4 in ACLOCAL_AMFLAGS so ./autogen.sh just works

[ben: commit message modified by]
Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
diff --git a/Makefile.am b/Makefile.am
index 0dd615b..20bca79 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -19,7 +19,7 @@
 #  IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
 #  CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
 
-ACLOCAL_AMFLAGS = ${ACLOCAL_FLAGS}
+ACLOCAL_AMFLAGS = ${ACLOCAL_FLAGS} -I m4
 
 SUBDIRS = lib man tools scripts benchmarks demos